To those of you who don't wear helmets - horses can trip and fall, too - even the bombproof ones. It's not just spooks that make people fall off.
Last summer I saw a really bad one - little girl on a bombproof pony, good footing. Pony took a bad step while they were trotting and flipped over. He rolled right over her. She had a seizure while she was lying there in the dirt, went to the hospital, but was ultimately fine. Now if she didn't have a helmet on... you can imagine what the outcome would have been.