A trope from a message board I often frequent:
When a character has the Idiot Ball
slipped into their pocket while they weren't looking, causing them to forget to properly use their abilities, intelligence, or powers to stop a badguy or get out of a situation, even though they may have used the ability in similar situations before(often many times). This happens often with Superheroes.
Of course, the only excuse for this to happen is that if the character didn't forget, the story would be over right then and there.
Because of that, the trope was given the name Plot-Induced Stupidity