<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by letiole:
I've heard setting your credit card on the bar scanners that are built into the counter at places such as Home Depot will also potentially demagnatize your card. I've had to call and ask for new ones. On my much-loved Amex, the signature strip is coming off from over use. 
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I think it's not the bar scanners, but the anti-theft deactivation thing that kills the cards. Some retailers have a metalic tag on high-value/frequently stolen items. The device which deactivates the tag has some kind of magnetic field which can also kill credit cards.
cnk