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.
I despise it when a character has had a long arch proving their worthy of what they do, and then it turns out late in the game they’re a chosen one or some shit. If you’ve been successfully fighting monsters for 15 books, going from a moderate combatant to a super mega awesome fucking wizard who wipes out an entire fucking species to save someone then you have proved your badass monster fighting chops, and you don’t need to be the chosen one. What made you awesome is that you were a (mostly) normal dude who became amazing through hard work and sacrifice. Now you’re just someone the gods chose or whatever and it completely ruins the entire concept of what the character was.
Two of my absolute favorite series of all time just recently did this, and I am devastated.