I haven't had too many "crazy" things happen at shows I've attended, just the typical stuff.
Probably the most memorable was actually at a show I was working, not riding in. There was a younger girl, maybe 9 or 10, riding this little palomino. They were both turned out pretty well, cute pair.
Well the pony took two steps into the arena and took off. The little girl, bless her, was a complete beginner (should NOT have been jumping, she was barely comfortable at the trot) and had no idea how to handle a run away horse. She managed to hang on a respectable time, but eventually fell.
We discovered later that the pony had actually been a barrel horse previously. He had been badly trained and was of the habit of rushing out of the gates and had a tendency to run off with his riders. Turns out they had just recently decided to make him a hunter/jumper and this was his first English show. No one had bothered to work him out of his past issues, apparently just thinking they'd go away on their own. *facepalm*
Sometimes I'd really like to know what goes on in people's minds.
Riding: The art of keeping a horse between you and the ground.