I'm leveling a holy/shadow priest, and the only real horror story has been Gnomeregan. I had a hard time with Stratholme, due to visibility issues and overly aggressive pulls, but Gnomeregan was just pure stupidity. The group was me in holy, 2 prot paladins, a prot warrior, and a rogue. All three tank spec'd players decide to tank. With my only healing spell at that point being flash heal, and all 3 tanks running around corners, we wipe pretty quick. The "dps" paladin leaves, and we get a warlock. Another wipe or two from tanks running ahead, and leaving the warlock to try to protect me, and we get past the first hallway. We get to the point where we jump down, and they all run right to the edge, and jump. Nevermind that none of them had more than 20% health before they did so. Those that survived the fall are swamped with mobs, and we wipe again. Running back to that point, someone takes a wrong turn, and after fighting off another swarm of mobs, we repeat the jump to our deaths. The warrior leaves, and is replaced by a hunter. We run back to the jump, and all make it down safely. However, the hunter's pet aggro's another swarm of mobs, and just as we're on our way, we're swamped from behind. Another wipe. Finally, the tank decides to just run forward, and leave us all behind. He dies repeatedly before we finally make it to the last boss, and somehow defeat it.
Though the good part of it was the warlock and the rogue thanking me for healing under such horrific circumstances =). The rogue and I were consistently the last to die in every wipe.