I have several actually. I work at a summer camp and the counselors get into shenanigans on horseback that usually end with people falling off hilariously.
1. At the beginning of the summer, I was up in the giant pasture with the oldest, most advanced kids. We had all taken turns racing on this flat part, so I asked another wrangler if he wanted to. We're galloping and both horses start to turn and veer towards the group over to the left. We pulled them back. Abby stopped so hard that I bounced in the saddle, thoroughly hurting my lady parts, which all of the kids (all girls) thought was just the funniest thing.
A minute later, I asked another wrangler to race. Both horses start to turn in the same spot again, so I stood up a little when Abby was going to stop. Nope. This time they both turned really quick and both me and the other wrangler just went flying off. I landed on my back with Abby looking at me like "What are you doing down there?" And all of the kids laughing their behinds off. We got up and hugged each other and laughed for the rest of the day.
2. This was right before we left camp for the summer. Me and two wranglers were riding around the arena ourselves, one being the one I fell off with above. We were trading horses around to see what they were like. I rode her horse that I had never ridden before in the saddle she was using. Except her legs are much shorter and she likes her stirrups high anyway. I became a jockey on an Araloosa. Turns out he's super sensitive to seat cues too. Glad that was mentioned.. I adjusted my butt in the saddle and leaned forward too much. He's off. Not cool.
So we're flying around the arena and when he gets back to the little box at one end, he stops. I was warned that he hops a little when he stops, so I expected that. But since my feet were so high, it didn't work. I bounce off and slip my elbow around the top post and the rest of me comes with so I just looked like I was standing on this fence resting (both elbows over the top post and standing on the bottom one) holding the reins still.
3. One was like Jamzimm's: This jerk horse I don't like was being ridden by a friend of mine. He didn't tell the horse to stop in the arena, so he kept going until he got to the fence, then stopped. My friend went flying forward, grabbed the fence and flipped himself over it so he landed standing on the outside of the arena.
4. The last one I can think of was a couple years ago. We had a girl who wanted to be a wrangler so badly, but she just wasn't a rider. We felt bad, it just wasn't meant to be.. She was riding with some other staff one day and we have no idea how..but she just slid off her horse. She was on the slowest, most dead-headed animal we have and she just slid off. We were walking by the way. No idea. It was the funniest thing and we still mention it at the corral often.