First off I want to say that this isn't a thread bashing LFG. I like it. This is a thread to vent about your worst experiences in a random dungeon group.
So today I decided to level my mage. I queued up for a random dungeon while I was questing and I got Mogu'shan Palace with a level 89 tank. Right before the second boss he hit level 90. The four remaining lower level people all erupted joyously for him! Seconds later he runs through all the remaining trash, pulls the boss, and then drops group. There were no survivors. Except the dungeon mobs. They all lived.