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.
People getting shot with a shitty handgun and they're dead as soon as they hit the ground. Even if its a fatal shot, chances are quite high you're going to die minutes or hours or days later if you make it to a hospital.
People hiding behind cars from bullets. Bullets being shot at the car and somehow not hitting them. Only the engine block could stop most bullets.
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People shooting guns in a car and then continuing their conversation...
You would be deaf.