Hit upon this today with a group of four. Believe the game figured it was just going down the vertical positions of the screen but, it ended up dumping one of us a level below the mountain.
Hopefully the screen shots will make it more clear. See how lalwphotography is put below the cliff when he was above it before transitioning?
I would think this means he'd have to take the long way back up but no just exiting the screen again would put him on top of Semos Mountain again. It's very easy to replicate with enough people.