I'll go first. Mine is the instant knockout drug. Like Dexter's intramuscular injection that causes someone to immediately lose consciousness. Or in the movie Split where there's the aerosol spray in your face that makes you instantly unconscious. Or pretty much any time someone uses chloroform.
When a story starts to bring in prophecy as part of the writing. As soon as a character does something "because the prophecy speaks of...", I feel that the writers ran out of plausible ideas and use that as a cheap crutch.
Battlestar Galactica was a great show, but they should've skipped that part.