This has happened to me twice. Once at a club, I was in the men's room taking a leak when this girl came out of the stall. I didn't even know she was there until she came out. She saw me and realized her mistake and was mortified. She was pretty drunk and probably desperate which I can see why she could make that mistake. The second time, I was in the men's room at this local sports bar and this guy came in with his girlfriend. The line to the ladies room was long so he took her into the men's room and had her piss in the stall. That was pretty hot.