Earlier this summer on one of my NW flights I was sitting in 1D when something happened back in Y. I could just tell by the F/A's body language that something was going on but I wasn't sure what. I saw her take several blankets back to Y and a few minutes later a man comes up to F and is standing in the galley area. The F/A appears to be apologizing to him and offering him a drink, etc. She tells him he can sit in the jumpseat (where he stays for the rest of the flight). A few minutes after that, the F/A says something to the guy sitting in 1C to the effect that she's sorry but the Y passengers are going to be coming by to use the lav in F because a passenger in Y had an "accident" and they need to sit on the lav in Y for the rest of the flight.
I didn't find out exactly what happened but I'm guessing someone's colostomy bag broke.