Mayonnaise wasn't the trick for removing permanent marker

[snopes]

There is an urban legend going around about how hotel key cards contain personal information including credit card numbers. The legend states you should always take the key card with you and destroy it rather than leaving it in the hotel room when you check out.

This urban legend is FALSE. Hotel key cards only contain room number lock codes -- nothing else. The next time you hear an urban legend like this, check it out on www.snopes.com before you believe it. Snopes is the best source on the web for verifying urban legends.

http://www.gainesvilletimes.com/news...s/161057.shtml