Originally Posted by spsawant

That's not the right analogy. The correct analogy would be as follows:
The Phone Cards were always advertised at the right amounts, it's just the Cashier charged you 4 cents instead of 500$ and the Customer who knew about the error decided to keep those Phone Cards which really amounts to stealing. Yes you do have a Receipt showing 4 cents as the Price of the 500$ Phone Card, but you knew all along that the price was 500$ and still decided to keep it as the Cashier did not realize his mistake.