I'll start. I usually tank heroics but decided to heal recently (that was my first mistake), and I landed a group of guildies from another realm in a Well of Eternity PUG. Now, the average Cataclysm heroic isn't all that tough to heal these days as long as you're sensibly geared, but it didn't take me long to realize that this group was blowing through an unusually large percentage of my mana pool. They stood in front of the Dreadlord Defenders' Carrion Swarm, couldn't find an interrupt button with two hands and a guide dog, and seemed to DPS at an unusually slow rate even with the crit buff given by Illidan's Shadow Walk.
It was around the time I noticed most of the group sitting in Peroth'arn's Fel Flames that it occurred to me that either this was the most legitimately incompetent group I've ever had the misfortune of encountering, or they were doing it on purpose. But because they never quite managed to get themselves or myself killed, I let it slide. I left at the end with 50 gold and a Forest Emerald from my Satchel, wishing for a Dungeon Finder system sufficiently advanced to recognize that some groups are definitely worth, say, a pony.