10 Biggest Doctor Who Retcons
8. Nobody Remembers That Aliens Exist
Ever wondered why everyone in Doctor Who is always really shocked to discover that aliens actually exist, despite the fact that Earth has been invaded countless times? The answer lies in Series 5's story arc, and it's essentially one giant retcon.
The cracks in time and space which recur throughout Matt Smith's first series as the Doctor have a dangerous tendency to erase things from existence, meaning nobody can remember them (aside from the Doctor, being clever and all). It happened to Rory during his encounter with the Silurians, and apparently it happened to many key events from the previous four series as well.
This is first revealed when the Doctor realises Amy doesn't recognise the Daleks, despite them invading her home planet at least twice in recent years. The Doctor also realises that there is no mention of the Cyber King stomping all over Victorian London in history books: another event that apparently disappeared down the continuity wormhole.
For all intents and purposes then, these events never happened. It's a pretty smart way of giving the new series a clean slate, but it also makes the carefully crafted world building of Russell T Davies' tenure feel a little inconsequential, which is a huge shame.