Microsoft points are the digital currency you use to buy Xbox and Zune content, and you can buy cards that are pre-loaded with points to enter in to your account. Problem? Just like with the iTunes gift cards, somebody discovered the algorithm Microsoft uses to generate valid codes, and loosed it onto the internet.
People were able to use the code-generation website to make about 1 million in Microsoft point gift cards, along with other Xbox items, before Microsoft stopped it. It's a hard problem to solve, because Microsoft probably needs to go to a completely new algorithm in order to stop it. [Tech Game via Escapist via Save and Quit via Joystiq]