I remember reading one of the "hacking exposed" books (the exact one I can't remember now), where there was an incident where an online store used values in the webpage itself to handle the prices of goods. In the example in the book, the hackers had replaced $45 list price with $1 giving huge discounts to the purchasers. And to make things even worse, some of the hackers would order dozens of shirts (at $1 each) and then return them for full price.

It might have been an earlier edition of this one, but I do remember the bright red cover: http://www.amazon.com/Hacking-Exposed-Web-Applications-2nd/d...