You drove and pulled up to the gas station to fill your gas tank. You exited your vehicle, opened your gas tank cover, grabbed and put the dispenser nozzle in. You also had to pay at some point.
How can you forget that the hose is still connected to your car when you drive off?
Depends, if it’s prepay by credit card you only get billed for amount used regardless of how much you tell the cashier. If you prepay by cash you have to go get change.
Most pump transactions are just done by credit or debit card and you are billed for actual use.