All I ever want in life is for an ATM to spit out an extra 20 dollar bill one time. This never happens! Vending machines have given me two bags of chips. I’ve gotten an extra chicken nugget here and there. Sometimes even the glorious onion ring in an order of fries. But the cash machine never messes up for me. Here’s how it manages to do that: with a series of rollers—some serrated, some smooth—that send the bill to a conveyor belt to get scanned with a sensor.