I fell off once doing a barrel pattern, but it was because we were coming after the last barrel and my horse just started bucking, and it was unexpected, and oddly enough I landed on my feet first and then my butt. And then there was one time he and I were cantering around our arena and a horse in the pasture started doing laps around the arena (our pasture surrounded our arena), and Rocket stopped and started bucking, and I held on for a little while, but it was like someone had turned him into a bronco! I finally came off, did a flip, and landing in the squatting position in front of him on my feet, reins in hand. Rocket stopped and was like, "Erm... how'd you get there?" xD
If you're not practicing, someone, somewhere is, and when you meet him, he will win.