The party of heroes just defeated the Big Bad. He's finally dead. Hurray! Now we can all go back and live peacefully.... BUT WAIT! During that final encounter with the villain, they completely forgot to deactivate the self-destruct mechanism he activated. It's not that they couldn't, but they just forgot. Cue the Oh Crap moments. This is mostly a comedy trope, though sometimes played seriously. Sometimes a disaster *COULD* be averted if someone had just remembered it. See also Your Princess Is in Another Castle, for times when it's the audience who realizes that it isn't quite over yet.