One, and it was an equestrian helmet during equestrian pursuits. My horse tripped on a cross country course, and I went off over her shoulder, landed on my feet but the forward momentum pushed me forward into a somersault and I planted my noggin into the ground. When I got up, it had cracked like an egg all the way through, held together only by the nylon webbing of the harness system.

My bicycle helmet just fell apart, this time it was as I was getting ready to go home from work. As I snapped the chin strap, I felt a sudden loosening around my head, and several of the straps just disconnected from eachother and the helmet. Since it was a cheap $15 helmet and I'd had it for two years, I sucked it up, and stopped at the LBS on my way home and got one of these: