I did a slow-mo head plant because the rear axle broke while I was starting to crank across an intersection. There was an ambulance waiting at the light. I took my Bell V1 Pro off and checked it for damage. The foam shattered, but the hard plastic shell was fine. The ambulance drove away unconcerned, but somebody else stopped and asked if I was OK.

All other helmet failures have been related to strap/buckle failures.