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 worst is when a show or movie establishes that X can’t be done, because Y. Then in a later scene X is done without addressing anything about Y. It’s actually pretty common, especially when run time needs to be padded with a side quest.