I had a Mini-Bike back in the early 70's and hurt myself a lot on it, between whacking tree branches (with my face usually), burning the inside of my leg when I dumped it after hitting some huge ruts made by off roaders the day before, and when I just plain screwed up and fell off.
One day, a friend and I were riding, both of us wearing helmets, and we were doing what we always did pretty much, riding up and down a gravel road, and going up and down a steep hill. I went down the hill, going about 40 by the time I hit the bottom, and suddenly, I was in the air, head at the bottom. I don't remember the hit at all, just remember the horrible pain in my legs (from the gravel) and my chin hurting like hell. The wind was knocked out of me too. My friend pulls up and I say something like "What happened?". He smiles, because he thought I was dead.
My chin hurt because the strap had snapped in two. I had a huge welt under it from the buckle snapping me. The helmet was still on my head, but was cracked into three pieces from the impact right at the top dead center. After a couple of minutes, my neck started cramping up. I have arthritis in my neck now, and this 'wreck" probably had something to do with it.
What caused it was the return spring for the rear disc brake had snapped in two, and the brake went to full lockup and I went airborne. After that day, I had two springs on it as a back up, and replaced both every 3 or 4 months. I still had friends who rode without helmets, even after I showed them my old broken helmet. Morons
