On the map “Blood in the Darkness”, from the The Curse of Drachenfels DLC.
In the corner, left of the exit, in the boss-room, my friend got pushed through a wall. He ended up stuck there with his only choice being jumping into the abyss.
How it happened:
My friend sat in the corner and two other players walked against him for a short while. After that he was on the other side of the wall.
So far it only happened once, and we didn’t think of making a screen shot, due to the oncoming horde.