In the December issue (vol. 50, no. 12) of the Communications of the ACM Hal Berghel takes a closer look at unsuspicious cards with a magnetic stripe on it, like membership cards, gift certificates or hotel room keys. He found the duplicated data of credit cards.
In the article Berghel describes the layout of the magnetic stripe and the data formats of its three tracks. He goes on and describes the development of a magnetic stripe anomaly detection system, called CardSleuth. This tool rings an alarm if a card doesn't actually contain what it is supposed to, like the hotel room key that holds credit card information.
The article is available through the ACM Digital Library (subscription required). Also there is the author's version which is available free of charge.
