Coaching the bases I don't wear a helmet, but our park requires them if you are in the cages.
My 9yo managed to hit a ball that I'm pretty sure defied the laws of physics in the way it deflected off a support pole and hit me square in the side of the head. It knocked the helmet (well, skullcap really) off of my head and sent me down to my knees where I got to look at these cool color bursts in front of my eyes for a few minutes.
If it wasn't for the helmet, I'm sure I would have gotten my dream of an early retirement fulfilled. Although I must admit having a nurse spoon feed me my lunch wasn't in my plans for at least the next 40 years or so.