You need to fill your gas tank and stop by a station that has an employee who does it for you. You tell him how much you want and you just wait in the car. A couple of minutes later the service is done, you have a look at the pump, and you pay the amount displayed; if you’re a nice guy, you might even give him (or indeed her) a tip. Where is the scam?

If you don’t get out of the car or if he notices that you haven’t looked at the pump before he starts filling, he won’t set the pump to 0 (from the last client) before he starts. The total will therefore include the amount from the previous sale, which has already been paid by the last customer.

Because most people know roughly how much they pay for a fill, the attendant will most likely pull the scam when the previous purchase is not very significant (eg. $10).

How to avoid: look at the pump clock as soon as he offers his services or, even, better, do your own filling.