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 Walking Dead (tv series at least) is a great example of inconsistency undermining the overall rules for their world. Instead of the danger of the dead overrunning everything from outside, the danger of the recently deceased causing an outbreak in any sizeable community was a far more interesting threat in that setting. But they only did that for a little bit and went back to the overwhelming masses of dead and ‘people are the real monsters’ over and over.