I was doing a coop mission with my ladyfriend, when I noticed that her icon on the minimap was beyond a wall. Via chat she told me that she is behind the wall indeed, and cannot come back into the corridor where that happened.
She plays a melee crusader and has no idea how she got behind the wall, except that it was during combat with a lot of enemies, so it could(!) have been the jump of her assault armor, but again, she is not sure about it.
When my avatar arrived on the scene, her map icon was still beyond the wall, but her name floated in the corridor and combat effects like her shield smash where displayed in the middle of the corridor.
The only thing we could do was to let me finish the mission alone and then leave, which got her avatar out of this strange situation.