In college my friends and I would go to the driving range a lot. It was a 3 tier range with a golf ball dispenser on each level, all networked together. We figured out (by chance) that if we swiped our range card and then quickly selected the bucket size of range balls, it would dispense the balls before it updated the entire system with the new dollar amount that should be on the card. So we basically got back the cost of the card (~ $20) the day we realized the hack just by swiping over and over (a large bucket was $12). All in all we got around $400 worth of balls before they put in a new payment system.