Heroic Sindragosa 25, some idiot blocked half the raid at about 15%, had to leave most of them in and just burn. Got her down to 80k before I died and she went for happy hour on the rest of the raid. Only 2 healers and an ele shammy up at that point, so the shaman's lava burst immediately got aggro and he was 1 shot. Healer dots took the boss to 3000 hp. Worst wipe ever.
Shade of Aran, casual guild I was in finally was getting him to sub 10% after a month of attempts. 1% HP raid is wiping, 2 dps left (ele shaman and mage.) Mage iceblocks to avoid taking a magic attack, Shaman dies. Mage removes ice block, PoMs, pyroblasts Aran, boss has like 1k hp left, Pyro dot is ticking, mage dies just as Shaman ankhs, lightning bolt is in mid-flight...lightning bolt hits boss..."Evade" and Aran disappears at 1 hp with the shaman alive.
For me back in Vanilla WOW it was in BWL (BlackWing Lair) on the 2nd boss which was Vaelastrasz - The corrupt. This boss was nicknamed by several guilds at the time as the 1% Wipe Boss because most guild almost always wiped at the 1% with this one. I remember the first time we took him down and everyone just said F!@)-YOU! to this boss - was fun though.
was probably on hagara hc we went into the last phase (an ice phase) and our tank DCd and got killed by the ice waves. We managed to go through the phase and i started tanking hagara who killed me way to fast. Anyway we managed to nuke her to .5% or something when our last player died...