A sacrifice for the victory!
Xalit has a good question in the WoW LJ: which boss do you always die on? Being a Shaman, I'm pretty good at not dying (in boss fights where we don't wipe, at least)-- I'm far enough away from the battle to avoid crazy AoE, and the fact that I don't top either the heal or the DPS meters means I'm pretty far down on the aggro lists.
But even more common than me dying every battle is our main healer. He always seems to bite the dust during a battle, whether we win or wipe-- even on a trash fight, he's usually the first to go. And our guildleader, a DPS Mage, was the butt of a joke for a while in the guild-- we laughed that he had to die for us to beat the boss. If he didn't die, the boss didn't get the sacrifice he needed, and we didn't get the loot.
I don't know that it ever got to the point where we were actually killing him during the fight just to make sure a down happened, but I'm sure it was joked about. Are there any fights you've been to where, even with a victory, someone just can't stay alive for some reason?
