[SpringheelJack]

There’s a survey at your link asking people what they’d do in this situation. Results:

Fill up and drive away 66%
Tell the clerk about the obvious mistake 14%
Fill up, then tell the clerk 17%
Drive away and avoid the situation entirely 3%

[Domandred]

Honestly speaking I would probably fill up and drive away, BUT not because I'm being dishonest. I don't pay attention to the price of gas hardly at all. Maybe one in three fill-ups I'll look to see what I've paid. It probably could be broke the other way at $30.00 a gallon and I wouldn't notice till my card got declined and eaten by the card taking machine.

Second to that I would drive away and avoid the situation. Again not because of the price or anything but because I'd take one look at the line and see the place across the street empty and go there instead.

[the OlLine Rebel]

In this case, the last “Drive away and avoid” is the best answer. THERE WAS NO PERSON TO TELL AT THAT POINT!