Mine happened a few years ago when I was still learning to ride. I was riding with some friends and we decided to have a 'competition' running barrels. The only catch? We had to swap horses AND ride english! Now at that time I'd only ridden a few horses, and my current gelding was a longo strided, 16.3hh paint named Buster, and I'd never ridden english before. Not wanting to be a stick in the mud though, I agreed. I got paired with a little 12.3hh morgan with a rather bumpy canter. All was well as I trotted around the barrels, and I was going to stay at the trot since I'd never done barrels OR english before, and never ridden this pony until that moment, but after the last turn she started cantering. I pulled back (too hard!) lost my seat (english saddles arent as deep as western, evidently) lost my stirrups, and flew off of her side and faceplanted! I jumped right up and caught Velvet, but I was SO embarressed. My instructor was there to see the whole thing too- and the next day when we were bringing in our minis she came up to me in front of a much of friends with our 25" yearling colt and beamed at me saying 'Hey Allyson! Think you can stay on this guy?' They still tease me about it even now because I fell off for 'no reason!'
I HAVE A VIDEO TOO!