Forum:Doctor Who television discontinuity and plot holes/Major inconsistencies and retcons

In the course of a history lasting more than forty years, hundreds of stories set in the Doctor Who Universe stories have been created in many different media. While, broadly speaking, the series has maintained a coherent internal history of the characters and universe, there have inevitably been occasions where the programme has contradicted itself in both general terms and in detail. This was sometimes due to one production team simply not being aware of what had previously been depicted on screen, but also occasionally the result of a deliberate attempt to rewrite history.

The original Doctor Who did not generally attempt to retcon away these inconsistencies on purpose, mainly because for the majority of the audience it would be only a trivial or non-existent issue. The only major and concerted attempt to retcon as it is generally understood happened in the late 1980s when the writing team assembled by Andrew Cartmel attempted to drastically reinterpret the Doctor's own personal history and status, an attempt which the cancellation of the series meant would never be properly realised.

In the 1990s many of the official Doctor Who stories that were released were written by fans of the programme who often attempted to address some of these 'inconsistencies' themselves, providing retcons where necessary. Retcons are more common in material other than the television series proper, but these stories also contain some of the most blatant inconsistencies. Ace, for example, has been given numerous different fates in different media, none of which are mutually compatible.

The Doctor
Later: The Doctor stole his TARDIS. (DW: The War Games)
 * Originally: The Doctor built his TARDIS.

Later: The Doctor has two hearts. (DW: Spearhead from Space) Retconned in expanded universe material: Time Lords acquire their second heart only after their first regeneration.
 * Originally: The Doctor has one heart. (DW: The Edge of Destruction)

Later: The Doctor has had a number of incarnations prior to the First Doctor. (DW: The Brain of Morbius)
 * Originally: The Doctor has not had incarnations prior to the First Doctor. (DW: The Three Doctors)

Later: The Eighth Doctor describes himself as half-human. (DW: Doctor Who: The TV Movie)
 * Originally: The Doctor has a fully alien (or as later explained, Gallifreyan) ancestry.

The Time Lords
Later: Time Lords have a set number of regenerations, which heavily implies that Time Lords have a finite lifespan. (DW: The Deadly Assassin)
 * Originally: The Time Lords can, as the Second Doctor said, "live forever, barring accidents". (DW: The War Games)

The Daleks
Later: The Daleks were the result of a thousand-year war and the genetic manipulation of the scientist Davros. (DW: Genesis of the Daleks)
 * Originally: The Daleks were the mutated survivors of a war fought using a neutron bomb.

UNIT

 * See the UNIT dating controversy.

The Doctor

 * The Sixth Doctor regenerated after his TARDIS was struck by the Rani's tractor beam. (DW: Time and the Rani)

Retcon: The (unborn) Seventh Doctor 'kills' the 6th Doctor to bring himself into being. (NA: Timewyrm: Revelation, NA: Head Games)

(further) Retcon: The Doctor was already dying when the TARDIS came under attack. (PDA: Spiral Scratch)

The Daleks
Retcon: The Doctor had destroyed the planet Antalin, rather than the real Skaro. (EDA: War of the Daleks)
 * The Seventh Doctor destroyed Skaro.