My personal worst accident isn't that bad. I was riding a horse I'd been working with, and it was winter. It was extremely warm that day, though, which was the only reason I was riding him. I didn't realize he would still be fresh, considering it had been freezing for endless weeks.
I hadn't ridden him in ages and was pretty tense.. I hadn't recalled how fast his canter was and freaked when he started to go. So I brought him back to a trot and then asked him on again... he took off at a full gallop, jumped through the air and landed bucking. Needless to say I went off and blacked out. O.o
Worst one I have seen/had happen was a friend of mine.. she was riding one of my old ponies, Sprout in a show. It was a pretty big show, and there were a loot of horses. Sprout is really nervous sometimes and they were all cantering in a group and he just got too scared. He bucked a couple times and she fell off. Her braces went through her lip & she was gushing blood.. fractured both of her arms as well. She has been riding again for a while but is off with tendonitis as a result of the fractures for the next two weeks.:(