One course I did I issued new Spesh helmets to all the delegates. We were sat in a circle while I talked about adjusting the helmet for fit, born on dates, etc, when one lass fumbled her lid and it dropped from her lap to the carpeted floor. She wasn't a tall person so the helmet didn't have far to go, yet it still cracked.
They are very easy to damage, and I wouldn't bet my life that the damage would always be visible or easy to see. Therefore, if you believe helmets work you should do what you can to minimise the chances of obtaining one that's been mistreated, or sent through the post multiple times as a return and put back into stock, etc.