Tenth Doctor's regeneration

The Tenth Doctor, who had previously regenerated and kept the same face during the Planetary Relocation Incident, (TV: Journey's End) met his regeneration into the Eleventh Doctor as a result of radiation poisoning intentionally incurred in order to save the life of Wilfred Mott after thwarting 's Final Sanction. (TV: The End of Time) By later accounts, this was the twelfth and final regeneration of the 12-regeneration life cycle beginning with the First Doctor. (TV: The Time of the Doctor)

Anticipation
According to one account, the Tremas Master claimed that the Valeyard was the Doctor's "penultimate reincarnation… somewhere between [their] twelfth and thirteenth regenerations". (PROSE: The Ultimate Foe)

After the Doctor and Donna Noble aided in the revolution of the Ood in 4126, Ood Sigma ominously warned the Doctor that "[he thinks his] song must end soon". (TV: Planet of the Ood)

During the Planetary Relocation Incident, the Doctor was fatally shot down by a single Dalek, triggering a regeneration. However, he was able to undergo the process without changing his appearance, by siphoning the remaining Artron energy into his severed hand after it had healed him. This later led to the creation of the Meta-Crisis Doctor. (TV: The Stolen Earth/Journey's End) By later accounts, this counted as the eleventh and penultimate regeneration of the Doctor's first regeneration cycle. (TV: The Time of the Doctor)

When he met Jackson Lake, a 19th century man who believed himself to be the Doctor after having picked up an infostamp, the Doctor initially assumed Jackson to be either his next incarnation or the "next but one". (TV: The Next Doctor) By a later account, however, he assumed the Twelfth Doctor to be his next and final incarnation, knowing his next regeneration to be his last. (COMIC: Vortex Butterflies) In an alternate timeline, in an encounter between him and his two immediate successors, the Tenth Doctor was skeptical of his twelfth incarnation's identity, believing "this man [could not] exist. Shouldn't exist!" (COMIC: Four Doctors) After he brought The 200 London bus and its passengers back through the wormhole from San Helios, Carmen, a passenger with precognitive psychic abilities, echoed the words of Ood Sigma, telling the Doctor that his song was ending, adding: ""It is returning. It is returning through the dark, and then Doctor...oh but then... he will knock four times.""

- TV: Planet of the Dead

After the Doctor's radical actions as the "Time Lord Victorious" led Adelaide Brooke to commit suicide, his sobering realisation that he had gone too far caused him to see a vision of Ood Sigma, to whom the Doctor fearfully asked if it was time for his death. After the vision had disappeared, the Doctor returned to the TARDIS and, following some contemplation, defiantly exclaimed "no", as he put the ship into flight. (TV: The Waters of Mars)

Shortly before his regeneration, the Tenth Doctor saw a solitary pale Watcher observing him; shortly after the TARDIS dematerialised, the Watcher also disappeared. (PROSE: The Advent of Fear) The Doctor encountered his successor in a dream, who assured him that they would be fine. (COMIC: To Sleep, Perchance to Scream)

As the Doctor parted ways with Mason Valentin, he admitted that there was something he had been avoiding, adding that, like the titular character of The Snowman, after all the "flying around, having fun, and doing the impossible", his time was up. (PROSE: The Doctor on My Shoulder)

Circumstances
When Gallifrey appeared in the sky above Earth, there was a tremendous kerfuffle. A man became trapped in the control booth, and Wilf freed him, himself becoming trapped. Once the Time Lords had been dealt with and the Master had died, the Doctor realised that the Nuclear Bolt had been left running, had gone critical, and was about to overload.

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Final reward
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Aftermath
The Eleventh Doctor's first priority was to inspect his new body, making sure he still had all vital body parts such as legs, arms, hands, and so on, before he briefly struggled to remember that the now-heavily damaged TARDIS was crashing. He enthusiastically threw himself into navigating the ship (TV: The End of Time) through the skies of London in April 1996, before he crashed into the back garden of a young Amelia Pond. (TV: The Eleventh Hour)

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Behind the scenes
Four different takes of the Doctor's final words were recorded, in which the emotion and grief was pushed further with each delivery. Said deliveries ranged from blunt and almost emotionless, to the Doctor being on the verge of tears. (CON: Allons-y!)