Retroactive continuity

Retroactive continuity is a narrative point established in a story which "fixes" continuity problems in older stories, or re-interprets past narrative elements in a new light. The shortened form of the phrase is retcon, which can be a noun or a verb. The act of providing narrative fixes for past sins is called retconning, a gerund of the verb. An individual instance of retconning is called a retcon.

Overview
Retcons happen in all major fictional narratives, especially those that have existed for a long time. Longer-lived franchises are particularly prone to retconning, as they tend to be created by a number of different artists, not all of whom may have an enyclopaedic knowledge of the franchise's history. Thus, conflicting accounts will inevitably arise and refactoring of information may be necessary.

Within the Doctor Who universe
The Doctor Who universe, having been created in a particularly chaotic manner by hundreds if not thousands of different creative talents in many media, is littered with continuity mistakes — and retcons to explain them away. Indeed, the very nature of the franchise as a narrative about a time traveller in a universe where "time can be rewritten" invites contradictory accounts. The existence of a drug named retcon as an in-universe concept within Torchwood narratives is certainly a wink at the ubiquity of the concept within the DWU.

Examples
Examples of DWU retcons are numerous and varied. Some of the more obvious include:
 * The last-minute Caves of Androzani explanation for why the Fifth Doctor had been wearing celery on his lapel since Castrovalva.
 * Revelation of the Daleks has the Sixth Doctor recall that, in his last meeting with Davros, Davros' ship had blown up whilst he was apparently still aboard, referring to the events of Resurrection of the Daleks, in which Davros is last seen aboard a Prison Station which is destroyed alongside the Dalek Battlecruiser that had docked with it. Later, Davros would feature a meeting between Davros and the Doctor in the interim between Resurrection and Revelation; the story ends with Davros boarding a ship which is then destroyed, though the Doctor reasons that Davros survived.
 * Time Crash's "explanation" that the TARDIS console room design changes over the years were the result of the Doctor changing "the desktop theme" — thus absolving the set designers and decorators of narrative responsibility for their occasional inconsistencies over the years.
 * Idris' insistence that the TARDIS doors were meant to swing out, but the Doctor chose to swing them inward, thus allowing for those occasions like The Eleventh Hour and The Ice Warriors, where the doors did indeed swing out.
 * The Tenth Doctor's revelation in The Stolen Earth that the TARDIS console was six-sided because it was meant to be piloted by six people, thus explaining why all incarnations of the Doctor couldn't always fly the TARDIS reliably.
 * The notion of the TARDIS as a living being, which gradually seeped into the series starting with The Edge of Destruction, giving writers licence to have the TARDIS behave as narratively expedient.