There are quite a few nasty tricks you can play on others in WoW, whether they deserve it or not. I like to think I'm a nice person, but I've done my share of "run in, get the named quest mob before anyone else can, run out" -- sometimes it's an easy way to finish a quest, as everyone nearby will fall on the mob in hope it becomes theirs. On the other hand, the excuse "everyone else is doing it" doesn't hold much weight amongst grown adults.
What are your tales of cruelty and horror towards other WoW players? What's the worst trick you've fallen victim of?