It was my own fault. The Ducati Streetfighter's tires were cold and I was carrying too much speed. As I applied the front brake the wheel immediately tucked and the motorcycle threw me unceremoniously over the handlebars. Somehow, I flipped and landed going backwards. As my tailbone hit first, the momentum then whipped my head back and slapped my Arai Vector helmet hard (make that very hard) onto the tarmac.
By the time I stopped sliding, I was so woozy I couldn't stand up, and had to close my eyes and wait for my senses to return. The fact that I'm writing this shows I have recovered just fine, but the back of my helmet had clearly taken a solid impact.
Arai helmets are legendary for their comfortable fit, and this is partly due to their available choice of interior shapes. The Vector's Intermediate Oval interior is designed for heads with a front-to-back dimension longer than the side-to-side dimension. Arai has developed their technology over three generations, so they know what they're doing. Once again, I have to thank them for their superlative protection; I didn't suffer a serious concussion. For those who choose not to wear helmets, please, reconsider.