Silverspear, that happened to me as well a few years ago. I was at the fair and showing some people that our pony was safe to jump on. Well, he was indeed safe, but my jumping was not, and I overshot and fell off the other side, to their abundant laughter.
Another really embarrassing one was a lot of stupidity on my part. I got the great idea into my head to take a book out and go sit on the horses' backs while I read. I'd done it earlier that day with nothing happening, so I thought it was safe. Just like closing your eyes, reading really puts you off balance. No biggy, me, absorbed in the book, pony grazing quietly. I absently slapped at a fly on my bare leg and BAM! The pony takes off like a shot. I, of course, had no idea what was happening so I flipped right off the back. I remember myself flipping through the air in slow motion almost, and then I landed on my neck and gave my head a really hard whack (I thought I'd knocked myself silly for a while there). Thankfully, the damage was minor and the effects wore off after a few days. It hurt a lot but if I didn't land so awkwardly, I bet it would have looked completely hilarious. This was on a totally deadbroke beginner safe pony, too. Accidents happen, and I'm smart enough to not try that stunt again.
Lesson learned: don't read and ride. It kills!