Multi-Doctor event

"Multi-Doctor Event" was a term used by the Twelfth Doctor to refer to incidents in which several incarnations of the Doctor interacted.

Consequences
Depending on the exact circumstances, multi-Doctor events could be a potential danger to the stability of time and space, primarily being an opportunity for a temporal paradox to occur. As such, the Doctor normally tried to avoid multi-Doctor meetings if it could be helped. (COMIC: Four Doctors) If a Doctor were to inadvertently end up in a time and place which they knew one of their predecessors would also be, they would often take great care not to get in their predecessors way or make their presence known. (COMIC: Time Trick)

On some occasions, the Laws of Time prohibited TARDISes from even making contact with their past or future selves. The Second Doctor once noted that TARDISes were specifically programmed to prevent themselves coming in contact with their past or future selves, (AUDIO: Daughter of the Gods) and, on another occasion, an alternate version of the Seventh Doctor had to resort to complex manipulations to send a message to his past self about how his presence in Colditz would alter history as the TARDIS safety protocols prevented the Doctor from just going back himself. (AUDIO: Colditz) By the time of his eighth incarnation, the Time Lords began to speculate that the Doctor or his TARDIS had developed an "affinity" to make contact with their past selves when the Eighth Doctor was able to freely visit his past selves, not knowing that Rassilon was contributing to the Doctor's efforts to visit his past. (PROSE: The Eight Doctors)

Multi-Doctor events were normally forgotten by all except for the most recent incarnation involved. This was said to be as a result of each Doctor's timeline becoming unsynchronised when such an event occurred. Only incarnations of the Doctor who directly interacted during the event were affected. For instance, the Eleventh Doctor came away from teaming up with his previous incarnations to save Gallifrey with his memories of the event fully intact, despite the fact he was unknowingly being aided by his immediate successor. (TV: The Day of the Doctor)

Earlier Doctors could still retain some recollection of specific happenings which occurred during multi-Doctor events. After returning from his role in the Omega crisis, the First Doctor retained a vague recollection of having met "a dandy and a clown" while he was away. (PROSE: The Empire of Glass) The Tenth Doctor was aware that he had married Elizabeth I, (TV: The End of Time) despite his immediate successor being present at the ceremony, (TV: The Day of the Doctor) and the Seventh Doctor was also able to recall teaming up with his future incarnations to save Gallifrey while talking with the Fifth Doctor. (AUDIO: Cold Fusion)

In certain situations, the memories of the most recent incarnation involved were known to have returned as they witnessed said events unfolding in front of their predecessors. When the Fifth and Tenth Doctor's TARDIS' collided, the Tenth Doctor knew how to prevent the ensuing temporal paradox because he remembered watching himself doing it as his fifth incarnation. (TV: Time Crash) When the first three Doctors took part in the Game of Rassilon, the Fifth Doctor was later able to recall what his other selves had been doing during that time even though he hadn't been present, but it required a conscious effort to recall exact details (TV: The Five Doctors, PROSE: The Eight Doctors). When the Fourth and Tenth Doctors were in the Cathedral of Contemplation at the same time, the Tenth Doctor gained the Fourth Doctor's memories of the situation a few seconds after the earlier Doctor had done something, allowing him to keep track of his past self's activities. (AUDIO: Out of Time) Before jumping through a time fissure that would land him in front of his tenth incarnation in 1562, the Eleventh Doctor briefly remembered what was about to happen when he saw the fissure appear. (TV: The Day of the Doctor)

When the First Doctor and the Twelfth Doctor both refused to regenerate at the same place at the same time, it created a disruption in time that pulled Captain Hamish Lethbridge-Stewart to the South Pole in 1986 rather than returning him to his own time at Ypres, Belgium in 1914. The Twelfth Doctor, after learning that the Testimony Foundation was not malicious, explained to the First Doctor that the situation was their own fault and undertook to repair it by personally returning the Captain to his own time, albeit while adjusting the time frame to save the Captain's life. The event caused both Doctors to decide to regenerate; however, the First Doctor would not remember it due to his timeline being out of synch. (TV: Twice Upon a Time)

The Brigadier's funeral
The First Doctor was aware that "all" of his incarnations would be present at the funeral of Alistair Gordon Lethbridge-Stewart and would behave themselves, though some would argue at the wake. (PROSE: The Gift) Those known to have attended included the Second, Third, Sixth, Seventh, Ninth and Eleventh Doctors. (PROSE: Shroud of Sorrow)

The tests of Time
The Father of Time, a distant, ascended future self of the Doctor, went back in time to put his first incarnation through two Tests, depositing the Doctor at 76 Totter's Lane for the first time as a reward for completing the First Test satisfactorily and thereby ensuring that his own timeline ran smoothly. The Second Test, meanwhile, saw the First Doctor forced to try and prevent the TARDIS from falling into the heart of a sun by working together with his second, third, fourth, fifth and sixth selves; at the First Doctor's instigation, the six Doctors pooled their temporal abilities as Time Lords to reverse Time within the control room. Once more satisfied, the Father of Time sent all the Doctors back to their own times and adventures, at which point the First Doctor very nearly understood the Father's true identity. (COMIC: The Test of Time)

Christmas dinner
The First Doctor once invited his first seven successors to join him for Christmas dinner. (PROSE: The Feast of Seven... Eight and Nine)

The Omega crisis
The Time Lords temporarily extracted the Second Doctor, followed by the First Doctor, from their respective time streams to assist the Third Doctor against Omega. (TV: The Three Doctors) The Second Doctor was summoned by Goth while returning to the Land of Fiction to recover his recorder. (PROSE: Future Imperfect) The First Doctor, however, was caught in a time eddy and so could only provide his advice. (TV: The Three Doctors)

Creative differences
At some point, the Doctor's first four incarnations had attempted to form a band, only for it to fall apart due to creative differences. (COMIC: Day of the Tune)

Sea Devil attack
After he accidentally awoke an army of Sea Devils who proceeded to attack a submarine in England, the Fourth Doctor unexpectedly came into contact with the Third Doctor, who was accompanied by Jo Grant. Adopting a new identity to protect the timelines, the Doctor helped his predecessor to defeat the Sea Devils, and made a quick exit before the Third Doctor could discover who he was. (COMIC: Under Pressure)

The Game of Rassilon
Lord President Borusa used Time Scoops to bring the first five incarnations of the Doctor, as well as several of their companions, to the Death Zone on Gallifrey as part of his plot to gain Rassilon's gift of immortality. However, the Fourth Doctor and Romana II were caught in a time eddy and so failed to arrive, nearly endangering the Fifth Doctor's existence before the First found the TARDIS, stabilising the Fifth's existence. After Rassilon granted Borusa the gift of immortality by confining him into the stone of the Tomb of Rassilon, the Doctors and their companions returned to their respective times and places. (TV: The Five Doctors)

Chessene's gambit
When the Second Doctor was captured by Joinson Dastari and his augmented Androgum associate Chessene, the Time Lords permitted the Sixth Doctor to rescue his past self, allowing him to travel to the space station where the younger Doctor had been captured so that he could trace his abductors. (TV: The Two Doctors)

Muldwych
The Seventh Doctor met a future incarnation who was known as Muldwych. Despite his concerns about the First Law of Time, the Doctor assisted Muldwych in defeating the Charrl. Shortly afterwards, the Seventh Doctor visited Muldwych and had tea with him. (PROSE: Birthright) Muldwych later attended the wedding of Bernice Summerfield and Jason Kane, giving the Seventh Doctor the book The Unformed Heart and two halves of a time ring to use as the wedding rings. (PROSE: Happy Endings)

The Hand of Omega
According to one account, the Seventh Doctor travelled back to give the Hand of Omega to the First Doctor. (PROSE: Echoes of Future Past)

Bonjaxx
During Bonjaxx's birthday celebration, the Seventh Doctor and Ace met a future incarnation of the Doctor, who came with his companion Ria. (COMIC: Party Animals)

The Key to Time
During his quest to assemble the Key to Time with Ace and Bernice Summerfield, the Seventh Doctor encountered his sixth incarnation while he was fishing with Frobisher. Discussing what difference it would make if they had not left Gallifrey, the Seventh Doctor was unable to convince his previous incarnation of their importance to the cosmos, but acquired his sixth incarnation's cat brooch, a piece of the Key to Time. Following this, the Seventh Doctor went to 1963 to acquire another piece, the TARDIS Instruction Manual. While doing so, he found his first incarnation, whom he tripped up to save him from an Imperial Dalek, which he shoved into the path of an oncoming bus. The Seventh Doctor quickly took his leave, leaving his original incarnation oblivious both to the identity of his saviour and to the danger he was in. (COMIC: Time & Time Again)

The Machine
A malfunction in an ancient prototype TARDIS on a human colony drew the Fifth and Seventh Doctors to the planet at different times, with the same disruption preventing the Doctors from simply returning to their TARDISes and leaving until the Machine had been destroyed. During their encounter, the Seventh Doctor began to recall the incident with the Knights of Velyshaa before being cut off by the Fifth Doctor, who had yet to experience that event. The Seventh Doctor also remembered teaming up with his other twelve selves to save Gallifrey. (PROSE: Cold Fusion; AUDIO: Cold Fusion)

The Sirens of Time
When the Sirens of Time manipulated the Fifth, Sixth and Seventh Doctors into changing history to give the Sirens more power, the Temperon drew the three Doctors together on Gallifrey after the Sirens had aided the Knights of Velyshaa in their attack on the planet, allowing the three Doctors to learn what had happened and take action to seal the Sirens away forever, apparently erasing these events for everyone but the three Doctors. (AUDIO: The Sirens of Time)

The Eighth Doctor's memory
When the Eighth Doctor lost his memory due to a trap set by the Master, Rassilon gave the Doctor subtle telepathic instructions so that he could still pilot the TARDIS. Under Rassilon's guidance, the Doctor visited all seven of his past selves at key points in their lives so that he could offer them advice and assistance, while the initial moment of telepathic contact restored his memories of everything that his past self had experienced up to the moment of their meeting, as well as gradually regaining memories of their future and his past. During these visits, the Eighth Doctor's initial contact with his past selves triggered a moment of temporal stasis that froze time for all but the two Doctors, allowing him to talk with his first and second incarnations without anyone else being aware of his presence, make plans with Third and Fifth Doctors, and rescue the Sixth from the Valeyard's attempt to force a new timeline to be created that would end with his execution. He also saved the Third Doctor from an attack by the Master, prevented the Seventh Doctor from being eaten by an Eight-Legs, gave the Fourth Doctor a blood transfusion after he was drained to the point of death by vampires, and helped Gallifrey deal with the political turmoil caused by the Sixth Doctor's trial (albeit operating in the background in the last case as he didn't officially exist according to Gallifrey's present). (PROSE: The Eight Doctors)

Gostak's Quest
At various points in their travels, the Fifth, Sixth and Eighth Doctors discovered one of three crystals containing the coordinates for a supposed lost moon of Gallifrey that had been a particular subject of interest for Gostak, the Doctor's favourite tutor back at the Academy. After the three Doctors found the crystals, they were brought together by Vakrass, the last of the ancient Death Lords, who wanted to stop Gostak's quest. Travelling through a portal to Gostak's location on the lost moon, the three Doctors learned that the moon was a complex temporal manipulation device that Gostak intended to use to undo the last few thousand millennia with the goal of re-establishing the Time Lords as a vast empire to control the universe according to his design. The Doctors refused to help his plans, with Gostak's efforts to force their aid being cut short as the Doctors turned the moon's power against itself, aided by the arrival of the Seventh Doctor with a control rod for the moon's system. The Doctors departed with their companions, leaving Gostak and Vakrass trapped on the lost moon as Vakrass kept Gostak where he could do no harm. (AUDIO: The End of the Beginning)

The Sirens' return
When the Sirens of Time attempted to escape their prison by diverting the path of a prototype TARDIS, Leela and Romana were able to prevent the crew succumbing to the Sirens' influence, but the captain of the proto-TARDIS subsequently killed himself and his crew to prevent them succumbing again. To ensure that the test flight of the proto-TARDIS still took place, Romana contacted Professor Bernice Summerfield for help, with Benny finding the Sixth, Seventh, Fourth, Eighth, Third and Fifth Doctors to act as substitute pilots while Romana served as the ship's captain. The First, Second and Tenth Doctors were also drawn to the temporal disruption, but departed to keep an eye out for potential side-effects rather than join their other selves once Leela and Benny affirmed that they had enough Doctors to deal with the problem already. (AUDIO: Collision Course)

TARDIS collision
The Tenth Doctor accidentally came into contact with the Fifth Doctor when their iterations of the TARDIS collided, resulting in the Fifth Doctor appearing in his successor's' TARDIS control room, the younger Doctor appearing older due to the presence of the two Doctors having 'shorted out the time differential'; in short, the Fifth Doctor was aged the amount of years he would have lived had he reached the Tenth's current age without using any regenerations. This caused a paradox which risked the creation of a black hole that would endanger the universe. However, the Tenth Doctor was able to quickly enact a solution, having remembered watching himself doing so from the perspective of his predecessor. Bidding farewell, the Fifth Doctor returned to his own time soon after. (TV: Time Crash)

Kotturuh crisis
When the Tenth Doctor, having declared himself Time Lord Victorious, began to rewrite history in the Dark Times by poisoning the Kotturuh, he came into direct conflict with the Eighth and Ninth Doctors, (PROSE: The Knight, The Fool and The Dead) leading opposite sides in the ensuing Battle of Mordeela. Afterwards the Doctors began working together to stop the Dalek Time Squad's schemes, with the Tenth Doctor eventually conceding his other selves were right that he'd gone too far. After they collaborated in the Defence of Gallifrey, the Eighth Doctor left his other selves to force the Daleks out of the Dark Times. The Ninth and Tenth Doctors reconciled on Birinji and then parted ways. (PROSE: All Flesh is Grass)

Cathedral of Contemplation
Whilst visiting the Cathedral of Contemplation, the Tenth Doctor meddled with the barriers separating each visitor’s timeline and encountered the Fourth Doctor during his visit to the Cathedral. The Doctors subsequently collaborated to stop the Daleks exploiting the Cathedral to win the Second Dalek War, tricking the Daleks into using a portal that took them to Earth shortly before it was destroyed as the sun expanded in the distant future. (AUDIO: Out of Time)

Saving Jago & Litefoot
At the end of his final reward just prior to his regeneration, the Tenth Doctor collaborated with his immediate successor to save Henry Gordon Jago and George Litefoot from the Gentlemen of the Dice. (AUDIO: The Jago & Litefoot Revival)

Stopping the Creevix
To stop the Creevix, the Eleventh Doctor sent messages to his previous selves. (AUDIO: The Time Machine) Via the TARDIS, he contacted the Sixth Doctor and Peri Brown to ask them to obtain an omniparadox from the Santa Maria on 12 October 1492, and store it in the TARDIS, (AUDIO: Trouble in Paradise) allowing him to escape from the Creevix. (AUDIO: The Time Machine) He sent a message to Susan Foreman via a radio DJ while she and the First Doctor were on Earth in October 1963. The message involved introducing Cedric Chivers to Bob Dylan. (AUDIO: Hunters of Earth) He contacted the Fourth Doctor and Romana whilst they were inside the Babblesphere, to make sure they didn't destroy it and sent it to the Stellaris Museum of Artificial Intelligence. (AUDIO: Babblesphere) Via the TARDIS Internal Communication System, he contacted the Eighth Doctor in London in 1935 to get him to clear up interference in the form of an alien invasion using the William Tell Overture to invade the Earth. (AUDIO: Enemy Aliens) Clearing the interference not only allowed the Fourth Doctor copy to lure the Creevix to 2013, it also allowed the Eleventh Doctor to send the messages to his other selves. (AUDIO: The Time Machine)

Using his psychic paper, he sent a message to the Second Doctor in 2724 to ensure he saved Sophie Topolovic's research on the Quiet Ones. (AUDIO: Shadow of Death) Using an Ovid sphere, he contacted the Fifth Doctor in England in the 1920s to ensure that he delivered the Ovid sphere back to the Ovids instead of destroying it. (AUDIO: Smoke and Mirrors) Via a recording on a telephone, he sent a message to the Third Doctor to ensure he saved the therocite and sent it to Professor Reynart. (AUDIO: Vengeance of the Stones) In the form of a distress beacon of the Howling Jupiter and a video message left in the control room of the wreck, he told the Tenth Doctor and Donna Noble to stop the Wraith Mining Cartel, by locating Professor Merrit Erskine, obtaining his research and using it to make sure the existence of slaughter crystals on the planet of Death's Deal was made known to galactic authorities. He also told them to ensure that Lyric Erskine survived. (AUDIO: Death's Deal) Using the Voice of Stone, he contacted the Seventh Doctor and Ace aboard the Obscura in the 49th century to make sure that they saved Captain OhOne. (AUDIO: Shockwave) Hijacking a giant television screen in New Vegas, he told the Ninth Doctor to save the life of Police Chief James McNeil. (AUDIO: Night of the Whisper) The omniparadox allowed the TARDIS to escape and rescue the Eleventh Doctor from the alternate future created by the Creevix timeline. (AUDIO: The Time Machine)

The Ovids, impressed with Tegan, shared their knowledge with humanity in the far future, giving the Eleventh Doctor the technology he needed to use the therocite, left behind by Reynart in his lab, which was now being used by Cedric, to destroy Cedric's time machine. The research on the Quiet Ones was used to create sub-pulsar transmissions, which were used to lure the Creevix into seeking out Cedric in 2013 after the copy of the Fourth Doctor inside the Babblesphere lured them there. The Bob Dylan music led to Cedric meeting his wife, and when the time came for him to use the time machine to complete the final step that would ensure the Creevix timeline, he hesitated, giving the Eleventh Doctor the time he needed to stop the Creevix. Lyric Erskine was the mother of Guy Taylor, the Time Agent that the Creevix erased from existence in order for their plan to succeed, and her survival led to her meeting Captain OhOne, who was Taylor's father, and the pair conceived Taylor on their second honeymoon at the Memorial Hotel, founded by Mayor James McNeil. Taylor's restoration and arrival in 2013 was the final piece in erasing the Creevix timeline. (AUDIO: The Time Machine)

Adam Mitchell's revenge
After Clara Oswald was abducted by Adam Mitchell, former companion of the Ninth Doctor, the Eleventh Doctor put a plan into action, tracking Adam to his fortress where he had allied with. The Doctor then summoned ten of his predecessors, minus his war incarnation, who had all lost their companions to Adam. With the assistance of Frobisher (who had previously been contacted by the Tenth Doctor while Frobisher was still travelling with the Sixth Doctor and told to disguise himself as Peri so that he could more easily escape capture), the Doctors were able to set their companions free. When the Master revealed his intent to destroy the universe, Adam turned back and stopped him at the cost of his life. The Doctors and their companions returned to their respective times, but not before overlooking a memorial in honour of Adam's sacrifice. (COMIC: The Choice, Endgame)

Saving Gallifrey
When the War Doctor prepared to use the Moment to end the Last Great Time War by destroying the Time Lords and the Daleks on Gallifrey, the sentient weapon brought him as well as the Eleventh Doctor from the Under Gallery, into contact with the Tenth Doctor in Elizabethan England, where a Zygon incursion had been uncovered. After the three Doctors arranged a ceasefire between the Zygons and humanity in the 21st century, the War Doctor returned to the barn accompanied by his future selves, who had been let through the time lock. When Clara urged the Doctors to find another way to end the war, they got the idea to transport Gallifrey into a pocket universe on the last day of the Time War. To do so required the work of "all thirteen" of the Doctor's incarnations, up to the Twelfth Doctor. Following this, the Eleventh Doctor returned to the Under Gallery where, after seeing off his predecessors, made contact with the Curator, an incarnation from his future. He then had a dream in which he stood alongside his predecessors, all looking up at their home planet. (TV: The Day of the Doctor)

Phone call
Shortly following his regeneration, the Twelfth Doctor made a brief indirect contact with his immediate predecessor when he listened in on a conversation on the phone between Clara and the Eleventh Doctor. The Eleventh Doctor, just prior to completing his regeneration following the Siege of Trenzalore, had made a call to the future to convince Clara to stay with his successor. (TV: Deep Breath)

The Voords' gambit
After being betrayed by Clara Oswald, (TV: Dark Water), an alternate Twelfth Doctor expelled her from the TARDIS but was driven mad by his loneliness. Finding the Voord, he became their new leader but steadily became aware that this was not how events were to proceed. Manipulating a past version of Clara, the alternate Doctor brought her, his past self and his tenth and eleventh incarnations to the Voord City, intending to reprogram them to ensure his existence. Having deemed Gabby Gonzalez as unimportant, she was able to travel back to when she'd first met Clara and Alice Obiefune, warning their past selves of the danger. Armed with the foreknowledge, the six thwarted the alternate Doctor. They intended to celebrate this with crepes in Paris but instead found the Ninth Doctor and Rose Tyler in the cafe. (COMIC: Four Doctors)

Help from the future
As the Tenth Doctor search for information on the Vortex Butterfly, the Twelfth Doctor summoned his predecessor to a planet to give him the answers he sought. Aware that the Tenth Doctor would be unable to fully recall the meeting, the Twelfth had reinforced the temporal echoes to leave him with strong feelings that at least made him aware that Gabby Gonzalez was at the heart of it all. (COMIC: Vortex Butterflies)

The Type 1 TARDIS crisis
When tracking a mysterious white energy that was destroying space, the Eleventh Doctor crash-landed in Gallifrey's distant past. With his ship damaged, he tried to get away by cannibalizing a Type 1 TARDIS for parts, but he ended up trapped in the Void when the Type 1 malfunctioned. The Doctor tried to convince the Type 1 to take him back by linking himself to the ship's telepathic circuits, only for the Type 1 to perceive the universe as a chaotic place, using the Void to draw creation into itself, creating the threat in the first place. The Doctor's other incarnations were eventually drawn into the rift that this had created, but once all thirteen Doctors had been drawn in, they were able to talk the Type 1 into stopping by syncing its telepathic circuits to the circuits of the Doctors' own TARDISes, allowing the TARDIS to show the Type 1 that the universe was worth preserving. (COMIC: The Lost Dimension)

Refusing to change twice
Refusing to change despite being mortally wounded by the Cybermen, the Twelfth Doctor was taken by the TARDIS to Antarctica in 1986, where he came into contact with the First Doctor. (TV: The Doctor Falls) The First Doctor, who was nearing his own regeneration, likewise was hesitant to change, and his contact with his successor caused a timeline error which resulted in the appearance of Captain Archibald Hamish Lethbridge-Stewart from World War I, pursued by a glass avatar of the Testimony. After uncovering the nature of the Testimony, as well as changing history so the Captain did not die just prior to the Christmas truce, the two Doctors took their leave of each other, having been convinced to face their respective regenerations. (TV: Twice Upon a Time)

Stopping the Scratchman
The Thirteenth Doctor visited the Fourth Doctor whilst he was trapped in the pocket dimension controlled by the Scratchman, to give him a reminder of what he stood for and what he had to be in order to encourage him to hold on to his true identity against the Scratchman's power. Once her past self, Sarah and Harry were back on Earth, the Thirteenth Doctor visited the Fourth directly to muse on how important it was that they never give up on being the Doctor. (PROSE: Scratchman)

The paradoxes
When the TARDIS bought the Thirteenth Doctor back to London in 1969, during the time in which her tenth incarnation and Martha Jones were stranded there by a Weeping Angel, she was forced to investigate a new mystery that had arisen while avoiding her predecessor to prevent a temporal paradox. Though she was able to team up with Martha for a time whilst Graham, Ryan, and Yaz kept an eye on the younger Time Lord, the two Doctors ultimately had no choice but to come face to face. The two Doctors successfully thwarted an attempted Auton invasion by luring a Weeping Angel into sending a Nestene Consciousness back in time. (COMIC: A Little Help from My Friends) The two Doctors soon parted ways believing they had successfully avoided a paradox, only for the Thirteenth Doctor and her TARDIS team to return to 2020 and find it long since taken over by the Sea Devils, which the Doctor recognised as being a consequence of her previous adventure.

As the Thirteenth Doctor sought to correct history by travelling back to 1903, where events involving the Skithra’s attempted kidnap of Nikola Tesla had been rewritten, she came into contact with the Tenth Doctor again at a different point in his life. Together the Doctors corrected the timeline as parted ways, however shortly afterwards the Thirteenth Doctor realised there was still a problem with the timelines and rushed to her past self’s aid. (COMIC: Alternating Current) She helped him escape the Dalek Restoration Empire, after he’d become embroiled in the altered timeline created by the Kotturuh crisis, and the two incarnations went their separate ways. (COMIC: Defender of the Daleks)

The "Fugitive Doctor"
Whilst dealing with a Judoon invasion in Gloucester in 2020, the Thirteenth Doctor encountered an alleged past incarnation of herself who she did not remember; a woman who had been living under a Chameleon Arch as a human called "Ruth Clayton". Neither of the Doctors recognised each other, initially leading both to believe the other was an incarnation in their future. Realising that "Ruth" seemed to be from a time at which Gallifrey was still standing, this led the Thirteenth Doctor to the realisation that "Ruth" was indeed from the Doctor's past, due to Gallifrey having been left in ruins by by her time. (TV: Fugitive of the Judoon) The mystery of "Ruth"'s existence was ultimately revealed when the Doctor learned the truth of Time Lord history, and of her previous identity as the Timeless Child, during which the Doctor was revealed to have countless lives before her assumed "first" incarnation which she could not remember. (TV: The Timeless Children)

Near-misses
There were various cases where more than one Doctor dealt with a particular crisis without directly interacting with each other.

TARDISes linked
When the Second Doctor's TARDIS became trapped in a pocket dimension that disrupted the link to the TARDIS's interior dimensions and destabilised reality, the High Council gave the Fourth Doctor and Romana permission to go against the Laws of Time and establish a link between the Fourth and Second Doctor's TARDISes so that the Fourth Doctor and Romana could enter the Second Doctor's console room and bring the interior back into sync with the exterior. During this crisis the Second Doctor and his companions entered the console room while the Fourth Doctor was still in it, but he and Romana hid on the other side of the console from the Second Doctor and his companions, and carefully manoeuvred around the room to stay out of sight until the Second Doctor left again, at which point they returned to the future version of the ship. (PROSE: Heart of TARDIS)

American park
The Second and Eight Doctors once ran past each other, and a splinter of Clara Oswald, at an American park. (TV: The Name of the Doctor)

The Holy Grail
When the Eighth Doctor was recovering from his amnesia after the destruction of Gallifrey (PROSE: The Ancestor Cell), his TARDIS drew its past self off course so that Harry Sullivan would be available to assist the Eighth Doctor in dealing with the plans of Lady Hester Stanton and her son George, who believed themselves to be the reincarnations of Morgraine le Fay and Mordred. Harry and the Eighth Doctor were able to defeat Hester and drive George to a psychotic breakdown in November 1936, while the Fourth Doctor and Sarah Jane dealt with the aftermath of this crisis in December 1936. Once they had tied up the loose ends in December, the Fourth Doctor and Sarah travelled back to November to retrieve Harry, the Eighth Doctor currently unconscious and his amnesia preventing Harry or others from realising that the two Doctors were the same man (PROSE: Wolfsbane)

Sycorax invasion
Whilst the newly-regenerated Tenth Doctor was aboard the Sycorax spaceship over London during their invasion on Christmas Day 2006, (TV: The Christmas Invasion) the Third Doctor arrived in London responding to a distress signal calling for the Doctor's help, accompanied by Jo Grant and Mike Yates. The Third Doctor experienced a momentary energy drain as his future incarnation absorbed some of the Third's own regenerative energy to stabilise his new incarnation. Meeting Jackie Tyler, the Third Doctor learnt that the Sycorax were invading, but the Tenth Doctor defeated them before his past incarnation could contribute to solving the crisis. However, the Third Doctor was able to free the Master from attacking tinsel after he failed to make an alliance with the Sycorax. He, Jo and Yates departed as Jackie went to reunite with the Tenth Doctor. (PROSE: The Christmas Inversion)

Orriv
A multi-Doctor event was narrowly avoided when the Eighth and Tenth Doctors encountered the Network. The Eighth Doctor encountered it on a space station above Orriv, the population of which it had killed. The Eighth Doctor attempted to trap it on Orriv, but it escaped by duplicating itself forcing him to flee. The Tenth Doctor arrived on Orriv with a crew seeking the creature which had killed Orriv, and encountered it in ruins of the space station which it had crashed into the planet. The Tenth Doctor contained the Network after brokering a deal with it and returned to the crew’s ship, just as it received a message from the Eighth Doctor, now working with the Tenth Justice Fleet, warning them against coming to Orriv. After reflecting on the crises his younger self was about to face, the Tenth Doctor chose not to respond. (AUDIO: Echoes of Extinction)

Time Conference
When the Eleventh Doctor and Clara Oswald received a psychic invite to a Time Conference on Refkeet Nine, they arrived to find the First Doctor and Susan Foreman also in attendance, having received the same invite by mistake. When the invite turned out to be a trap, the Eleventh Doctor, making sure to not be noticed by his younger self, with Clara's help, freed a captive Nuppino horse - that was being "trained" to "teach [the Doctor] a lesson for his meddling in time" - which ran amok, giving the hosts a scare. The Eleventh Doctor and Clara slipped away as the First Doctor sharply berated the hosts for their poor treatment of the horse. (COMIC: Time Trick)

Coronation of Elizabeth II
As witnessed by Eva De Ville, the first thirteen numbered incarnations of the Doctor and their various companions were present in London for the coronation of Elizabeth II on 2 June 1953, several while dealing with alien incursions, but managed to avoid coming into contact with one another. (COMIC: Where's the Doctor?) This included two versions of the Tenth Doctor; one dealing with a large, unidentified creature with Donna Noble, as witnessed by De Ville, and another dealing with the Wire with Rose Tyler. (TV: The Idiot's Lantern)