I'm pretty sure that helmets designed for a one impact life.    It sucks, but it's the reality if you want 100% out of a helmet.    One good hit to the helmet, you should get a new one.   Obviously that is totally unrealistic, but them's the facts.
My helmet is in the tail end of it's second season and I've definitely taken two or three good hits to the head while wearing it.   I was talking the other day about having to replace it.  I'll probably wait till the fall though.