The bug occured in level 8 where you get teleported in a large room full of herders.
I took the teleporter and I spawned in the room at the exact same time a herder was steping toward my tile, so he became impalpable and I shared the tile with him (it was pretty funny :p). I moved away, and I still could pass through him.
I tried to take a screenshot with "printscreen" but I ended up with a black screen -_-