Here's one that was downright mean.

Back when cutoff jean shorts was the fad, it used to be the big joke to light the little threads of someone else's jean shorts on fire. Many times someone would light these little guys and they would sizzle and burn for thirty seconds beofre the person realized and freaked out.

One day the guy who was notorious for lighting eveyone's short threads was wearing a pair with some long 'wicks' and someone had to get him, so my buddy goes over there and lights him up while he is bent over working on something. He doesn't notice and proceeds to find a friend of his because he has a present for him, he finds the buddy, bends over and let loose a fireball of flatulence, one that gave him third degree burns in the worst of places.