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.
The problem with this one is that, as a reader/watcher/whatever, it affects your experience even when it doesn’t happen. I was so convinced that Dumbledore was alive at the end of book six. Fell off a balcony? Point of view character gets dragged to the infirmary so we can’t see what happens after that? There’s a phoenix, a bird associated with healing and rebirth, conspicuously singing? That guy is pulling a Gandalf in the next book for sure.
So I spent the whole next book waiting for the dramatic reveal that never came…