There's only been one I can remember, and it was a Stone Vigil (normal) run with a tank who was literally incapable of tanking. Gear twenty levels below the dungeon, didn't have Paladin, couldn't keep aggro at all (and let me tell you, Stone Vigil Normal mobs eat your healer fast).

Four wipes before the first boss, and it was a unanimous ragequit from the remaining three of us.