This isn't as out there as a lot of the embarrassing moments here, but here's mine.

I was at a sleepover party and we were mucking around. I was being stupid and fell on someone, landing on my front. She was annoyed and I the way I landed and where I was, I couldn't get up right away.

She pulled down both my pants and underwear, leaving my bare ass exposed to everyone who were way too close. They were laughing their heads up as I struggled to get up and finally pulled my pants up.

Later that night someone spilt their drink and I sat down in it.

My ass wasn't having a very good night. :-/