I once read about a gas station that rigged their pumps. They discovered that the 'gas police' who came and measured gas to make sure pumps were properly working; always pumped a multiple of 10 gallons to measure.

They rigged the pumps to rack up more charges for the first 5 gallons, and then slow the charges for the next 5 gallons. That way it always read 10 gallons at those intervals, but overcharged you if you got less. Someone pumping 4 gallons (or 14) was charged as if they had pumped 6 (or 16).

They got caught when someone noticed that the gauge ran faster, then slower as you filled up.