There’s a serial called Mawdryn Undead that brings back the character of Brigadier Lethbridge-Stewart who, as head of the UNIT organisation, had been a major part of the show starting in the late sixties but stopped appearing in the mid-seventies, who got popped in because they couldn’t get the actor who played another character back to reprise their part. It was a seemingly innocuous decision, except that it blatantly contradicted when the earlier serials that the character had appeared in were meant to be taking place! The result is a debate that’s ongoing to this day among fans that have even popped up in throwaway references in the continuation of the series that’s been on air since 2005.