they make you pay after the fact? LEGALLY obliged, no moral discussion here (I understand many people, myself include, would feel it is the "honest" thing to do but I am talking legal). You can think of many different scenarios; your cellular provider accidentally credited money to your account, an mechanic forgot to add certain parts to your bill, or you went to the grocery store and the clerk incorrectly rang something up. Also if the error is the businesses error can they still come to you in the future and demand payment. Thanks!