<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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From a merchant.
We have had to refuse cards where the signature strip had worn off of CC. (the word void will magically appear) If the signature line is unreadable or reads void, the merchant has no recourse if AMEX (or any other CC) challenges the card.
The simplest procedure with AMEX is to call them. They will FEDEX a new card overnight.
As to the demagnetization. It often happens if the wallet is placed on the TV in a Hotel Room. Or if the Wallet is on the desk next to a laptop (or behind it)
Just a merchant and traveler's observations.