What is "that thing" in your fandom that spawned a whole category of fix it fics?

That one scene that the writers bungled, the one detail that was ignored, the plot hole that needed a solution, the misunderstanding that never got resolved. Whatever the case may be, what's that thing that inspired so many writers that it has it's own tag on AO3?

I'll start, in Supernatural season 4there's a scene where Dean leaves a brotherly voicemail for Sam, but thanks to supernatural interferrence Sam hears Dean threatening to kill him instead. The show goes on for over a decade after that and Sam never figures out that the voicemail was fake. Cue scores of writers saying "In no world can I let that stand" and writing a scene where it gets brought up.