When TV show creators get multiple seasons of a linear storyline under their belt the issues of continuity and retaining world integrity arise.
It's part of the creator’s job to keep track of everything that happened during the previous seasons in order to keep the storyline on track for future seasons.
This continuity is especially important in extremely linear genres like sci-fi, fantasy and drama, so what is a show creator to do when an episode goes against the show’s established storyline?
Generally they pretend that episode never happened and move on, and we’re left wondering what the creators were thinking when they came up with such a game changing episode!
io9 put together a collection of 10 TV Episodes That Everybody Pretends Never Happened, which includes the episode where Buffy is actually a mental patient and not a vampire slayer after all, my pick from the list for ultimate WTF TV moment.