db said:
A customer enters your cigar shop and purchases a $6 cigar. She hands you a $10 bill. It is early in the day, and you have no change in the till. You go next door to the café to get change. You return and give the customer his change and he leaves the store. Later in the day the café owner comes over and claims the bill you gave her was fake. She hands you the bill and sure enough, it is a forgery. You hand the café owner $10 and she leaves.
Now, other than the cost of the cigar, how much did you lose?