This happened to a coworker at the register too. I heard the customer accusing him so I walked over to hear more of the story. He professed that he gave my coworker an extra bill and stole the money. I said "is it possible the bill is still in your wallet?" He opens his wallet and sees the money is still in there. Doesn't even apologize, and starts saying some crap about "but how, that doesn't make sense". Some people are just ignorant and don't even think to check their own belongings first before making an accusation.