The vast majority of the time, WoW is just pure fun to play, and even nights spent wiping on a boss can be an oddly happy experience if you're running with a cool group of people and you feel like you're making progress. But we've all had that moment in a raid or a group where you realize that things have unstoppably taken a turn for the worse, and that there's not much (if anything) you can do about it.
I've actually had two of these in recent memory: being within one perfect Kel'Thuzad kill of server-first Immortal and then losing a single player to the second Frost Blast, and then having to call a Sarth 3D kill because people seemed to find an array of new and interesting ways to get themselves killed. The silence in vent on both occasions was pretty awful, and that these two nights occurred back to back probably didn't help anyone's frame of mind. I've had a Hunter buddy previously describe her worst moment as triggering the old Karazhan pet bug -- wherein you could aggro almost everything in the instance due to bizarre pet pathing mechanics -- when she was a trial member with a new guild. A Warlock friend still cringes when he thinks about the night he accidentally looted the Champion gloves off Curator, and then having to keep the group there for two hours while we waited on a ticket (back before any of us knew how loot transfers worked). Sometimes you realize the night's just going to stay bad, and that it's time to get out of Dodge. What was your worst experience in this vein?