yea, I agree with the "its just car car" thought line on this thread.
When someone does that, I usually take a second to look at the ding, and rub over it with a finger, making the point that I saw what the person did. If they apologize, and agree that they're buying that night at the bar, I'll live and let go. If they either ignore it, or even worse say some shit like "oh it's just a scratch, you'll live"....someones gonna "fall down the stairs." I know it might just be a scratch, but disrespect is disrespect. Such people are not usually in my company very long.....