Recently I was doing a half-guild/half-PuG Arcatraz run, trying to get my Sha'tar rep up for heroic Mechanar (free Badges of Justice FTW). It was all going very smoothly, up until the last encounter. If you haven't been, the Skyriss fight is more of an event than a straight-out fight. Four creatures spawn before Skyriss finally does, one at a time, although one of them is an awesome gnome who helps you with mad DPS, and you get drink breaks in between so it's not that bad. Anyway, we finally get to Skyriss himself. It goes pretty well for a while, although our Mage got double-mind flayed and thereby died. Eventually the tank (Druid) went down, and then the Hunter. At this point it's just me (Priest) and the Rogue left, and I've been mind-controlled. Rogue does a very impressive job staying alive, gets Skyriss down to 2% before dying, and then...the encounter reset, because I was still mind-controlled -- there were no "friendly" players left.
The trigger mob to start the event over eventually respawned, we fought the first mini-boss again, but then the gnome, who spawns second, refused to spawn! This halted our progress through the encounter and, after opening a ticket and waiting for a while with no response, we had to call it. It was pretty frustrating. A similar thing happened to me at the chess event in Karazhan last weekend. The guy who was controlling our king disconnected, so we lost; however, you can usually restart again after a short break. Not this time. We waited around for a while, walked in and out of the room and even the instance; no joy. Frustrating again. What have been the most annoying bugged moments for you?