The popular locker room gag of putting Tiger Balm (or some other heat inducing ointment) into someone's athletic supporter. The ensuing extreme discomfort of the victim can be quite hilarious.
Back in high school, my friends and I laughed all day long after giving a fireball to some jerk who always hassled the retarded kids. Seeing him stagger around the locker room, grabbing his dick and screaming "My balls, my fuckin' balls!" was priceless.