As a member of my college equestrian team, all seniors traditionally took a trail ride right before graduation. I was riding a mare that was supposed to be road sound...but really wasn't.
She spooked about halfway through, and lost her balance on the asphalt road that we were riding near at the time. I went over her neck, and hit my head hard enough to knock myself out on the pavement. Luckily, I only ended up with a concussion, and was still able to participate in my graduation three days later.
I firmly believe that, had I not been wearing my Troxel helmet, I might have ended up with a much more serious head injury. I consider myself extremely lucky, and will never get on a horse without a helmet ever again.