Well, I have two that are ranked 1a and 1b so I'll leave it for you to decide...

1a: Out on a date, she decides she wants to go to a hotel room for some privacy. We're getting hot and heavy and suddenly she sits up and demands that I "take her home". Confused, I sit up and ask what was the matter? As I sat up, I farted...adding further embarassment to the situation. Never found out why she freaked...

1b: I was at my girlfriend's apartment, feeling somewhat under the weather due to the flu. It didn't stop us from getting frisky and soon she's on top cowgirl style fucking my dick REALLY hard and fast. I could tell she was getting close to a big orgasm when, suddenly, this awful rush of nausea comes over me. I quickly pushed her off of me as I ran for the bathroom to puke my guts out.

rabbit

Once my wife and I were going at it in the bedroom. She was on her back with her ass at edge of the bed, and I'm standing up, beside the bed, fucking her. We were both on the brink of orgasm when suddenly the throw rug I am standing on slips out from under me, my feet go flying, I face-plant into her pussy, and fall to the floor.

We both start laughing so hard, but hurt from my fall and our denied orgasms. Luckily I didn't hurt the more important parts.

Well, my fiancee was jerking off on my face one time and the come all shot up my nose. Never let him live that one down.