Disclaimer, this happened a long time ago and not to me. Probably early 2000s. We're at a packed house party with what I guess you could say wasn't exactly a group of outstanding citizens, but that's besides the point.
At any rate, a guy and a girl go in one of the bedrooms to get it on. He's trying to do her in the ass, but there is no lube. After a second or two of searching, he comes across a tube of Anbesol. (For those of you that don't know, Anbesol is a numbing agent that you put on your teeth/gums when you have a sore tooth.) That'll work, right? Wrong. Hind sight is 20/20/
This guy is stoked. He's got something to lube up with, so they go at it for a few minutes. Lo & behold, a few minutes later this chick comes bolting out of the bedroom, completely naked, and heads straight for the bathroom. Unfortunately for her, with this being a packed house party, the bathroom is in use. She bangs on the door profusely, but to no avail. Poor girl ends up shitting all over the floor right outside of the bathroom in front of 50+ (?) people.
I don't know if that qualifies as the biggest party foul, but it was one of the best/funniest/nastiest that I've ever seen.