Charlize is an avid cinema fan. Since she watches films on DVD each and every week, she takes out a DVD contract with her local DVD store, and units are simply deducted against her prepaid account each time she makes a rental. One Saturday afternoon, she makes her way to the counter with the latest art house offering, but is told that she has no units left on her contract. She must purchase a new contract.

This strikes Charlize as a little strange, and she requests a print-out of her account, to satisfy herself that this is correct. The store clerk tells her that he is not allowed to provide customers with print-outs. Charlize insists, and finally secures a print-out of her account. When she checks this print-out, it is clear to her that she has not yet used up all of the units which she purchased. She still has…