I've heard of just that happening with a helmet before, but it was a motorcycle helmet (in a motorcycling situation). It was a one of those flip-up/full face helmets and the problem is that the jaw-protection is largely superficial and it cracks easily in an impact -- then the shards generally go straight into your pretty face. Here's a link of the type of helmet-style I'm talking about.
Flip-Up Helmets | Helmet City
You should tell WRSI just so they can think about it in their next generation of helmets. I have a Sweet Rocker helmet and it's visor could do the same thing, but I like it anyway.
BTW, I agree with Ture and others above. Helmets need to balance the factors of absorbing impact & protecting from sharp objects. I might look like Napoleon Dynamite in my creeking helmet, but it guards my grill and would absorb a hard hit versus sending it straight on to my already abused brain.
In theory river rocks are supposed to be round, too, right? Ha!