This isn't a short story collection, but it is.
A man accidentally opens a bathroom stall only to find that it's occupied. Miffed, the man says "Where I come from, we lock the door to the bathroom stall", and from there the game is on. What ensues are a bunch of short stories that are traded by the two men, one in the stall and the other out.
It sounds like a strange premise, and I guess it is, but I enjoyed the hell out of it. The stories were very cool (especially the one regarding the headless woman), strange and unique. I will be checking out more works by this author in the future.