A friends broke a vertebra(maybe more than one) when she high sided in a track with the back protector on. Yes, the back protector caused the damage.
Yes and no - the thing to remember is that this was a major highside and she had some other pretty serious injuries including two broken arms! The PLACE where the break happened was attributed to the back protector, but had she not been wearing one at all chances are the injuries could have been worse.