A customer moves away from a bank window, counts his change, and then goes back and says to the cashier,  “Hey, you gave  me the wrong change!"

"Sir, you stepped away from the counter," said the cashier. "We don't make corrections after you leave. There's nothing I can do about it now.  That's the policy of this bank."

"Well, ok," answered the customer. "Just thought you'd like to know that you gave me an extra twenty. Bye."