So inside Karazhan, there's a boss called the Shade of Aran. He's Medivh's father, actually-- or, more specifically, the spirit of Medivh's father. He's a crazy mage with all kinds of casting abilities, one of which is called Flame Wreath. Flame Wreath targets 3 random people in the raid. If everyone is spread out enough that no one is close to those three players, nothing happens. But if people are close to them, a ring of fire shoots out around the player, and anyone who crosses through it will cause everyone in the room to get hit for 3000-4000 damage.
For this reason: I will not move when Flame Wreath is cast or the raid blows up.
That's one of the best sounding WoW YTMNDs I've seen, but it's definitely not the only one. You could always check out Riker getting disappointed, Hello Kitty Island Adventure, a little Alliance vs. Horde DDR, or the Patch Day Simulator. YTMND is such a crazy form of expression, but it's no surprise, considering the two cultures, that WoW and their sites find so much common ground.
[ Thanks Markalept and Druid dude! ]