Occasionally, confusion is the most hilarious thing possible. Mix that confusion with fear in a styrofoam cup and you'll be enjoying yourself as you serve your friend a nice tasty prank. Put a little note under the windshield wiper with a super apologetic, but incredibly vague, statement about hitting the person's car but having to dip on account of your dog getting Crayola Crayon poisoning. With nothing actually wrong with the car, the person will be running himself crazy as he circles back and forth looking for what happened. Chances are, he or she will even find some sort of scratch that was there before and blame it on the phantom law-breaker.