The Doctor Who 10th Anniversary story "The Three Doctors" is released, and reveals time travel was invented by some guy named Omega who got himself trapped in a parallel universe made of antimatter. Bob Baker and Dave Martin massively retcon the series' history.
The Doctor Who 20th Anniversary story "The Five Doctors" is released, and reveals the Doctor's mentor Borusa has been a bad guy this whole time and that Rassilon, one of the founders of Time Lord society, is actually immortal. Terrance Dicks massively retcons the series' history.
The Doctor Who 25th Anniversary story "Remembrance of the Daleks" is released, and reveals that ever since the first serial the Doctor has been hiding in ancient super weapon on earth to use against the Daleks, and may or may not be one of the founders of Time Lord society. Ben Aaronivich massively retcons the series' history.
The Doctor Who 30th Anniversary story "Dimensions in Time" is released, and reveals the Doctor Who universe and the EastEnders universe are one and the same. John Nathan-Turner massively retcons the series' history.
The Doctor Who 40th Anniversary audio "Zagreus" is released and reveals all the novels actually happened in an alternate universe. Gary Russel and Alan Barnes massively retcon the series' history.
The Doctor Who 50th Anniversary story "Day of the Doctor" is released and reveals that Gallifrey was not destroyed by the Doctor but in fact tucked away into an alternate dimension. Steven Moffat somehow becomes the first writer in Doctor Who history to massively retcon the series' history on an anniversary.