Hate to but in, but I think it has! I've seen many jockeys that have fallen, but the horse is trained to keep running, not stop because your rider fell off! To the horse, you don't need a rider to race xD So I'm sure there has been a few cases where the horse finished without a rider. Though, sometimes it can be dangerous if the horse gets turned around and starts running the other way, towards other horses. There have been cases of head on collisions that lead to instant death or result in the horses having to be put to sleep. So..it's best if the rider doesn't fall off xD