The coming up short part comes in when they come back and try to re-do the transaction, saying they have exact change. Let's say the drink cost 69 cents. Cashier gives them the drink, takes the $20 bill, and gives them back $19.31. Everybody's even at that point. Short-changer then says, "Hey, I have exact change, so just give me my 20 back, and I'll give you exact change." Confused cashier gives them the $20, short-changer then gives them 69 cents, and walks away. The register is then $19.31 short. People try things similar to this at the betting windows all the time. You have to be alert, and stick to your guns, or you're going to get ripped off.