It's happened to me before. It was a few years ago, when I was really reckless and wild (smoking crack all night, taking loads of MDMA, drinking E&J for breakfast...). It was after a long night, I had passed out on a set of chairs that one can push together and it'll make a couch. Anyway, like others have also said, I dreamt that I was in the bathroom pissing.
But I wasn't -- I woke up halfway through my piss & tried to force myself to stop, and if any of you have tried doing that, you know it's not the easiest or the most pleasant feeling in the world. By the time I was able to finish, and run to the bathroom and back out...I had pissed all through my boxers, pants, onto BOTH of the chairs..and of course, the carpet. Everyone else was still passed out at this point. I took my boxers off, stuffed them in between the cushions, pushed the chairs back together and gave it a quick wipe-down with a paper towel. Then I left. No one ever said anything.
Whats funny, though, is the girl that was on the sofa next to me was semi-concious, but not enough to really be aware as to what was going on. When I was trying to hide my boxers (LOL), she moved around and had rested her hand RIGHT where I pissed all over on the chair. She mumbled something along the lines of "Why is this wet..? It makes no sense.." and passed out again