I have never had anyone respond to this with anything other than a very embarassed look and an immediate move to the back of the line. People don't get snippy, because you're graciously offering them a concession they almost certainly don't deserve. Instead they're forced to consider whether they really need to go first, or whether (more likely) it was petty and childish of them to push in. This approach also leaves room for goodwill in those rare situations where the person is in a rush because of a genuine personal emergency.