World of Warcraft. Raiding was canceled because not enough people showed up, so the 15 who did show up decided it would be fun to troll some allies. That's *troll allies* not gank them.... in fact we avoided killing them.
What we did was just swarm around people in tanaris and spam slows, polymorphs, hexes and whatever other slowing moves we had, and follow them until they decided to log out. we did this to about 6 people.
There was another time when a few of my guildees were running an "ulduar raid", but instead, summoned people using a warlock stone placed carefully in a spot where people would fall to their death (not everyone fell for it). We also posted the vent info for a rival guild, needless to say they were pissed when they figured out it was us x D.