I have to tell you this story, because it amazed me, showing how blind you can sometimes be to the obvious...
Recently my local ATM, when fed with my Maestro card, told me it would not recognize the card. Hmm, so let's try a different ATM - same result. So s.th. must have gone wrong with my card, after 2 years of usage without a problem. Well, WTH, I called my bank and ordered a replacement card. Which served its purpose for about 3 weeks before it, too, quit its duty. Now that was strange. Terribly unreliable cards they are making these days... So, another (slightly embarrassing, this time) call & fax to my bank.
Then, one day, it so happened that I was in my bathroom, and I had put my Jasjar with the nice XaX leather case (I wrote about it here) on the edge of my bathtub. I did not pay attention for a moment, and pushed it off the edge. It didn't fall inside, though, to my astonishment, but it stuck to the inside wall of the metal bathtub.
And then I hit myself hard on my forehead for being extraordinarily stupid. The leather case has a magnetic fastening, two strong magnets that prevent it from opening and the device from dropping to the floor. These magnets kept the device stuck to the bathtub. Nice, eh? But that's not the only thing they did - they deleted my Maestro cards' magnetic stripes, as well! Because usually, when I come home, I pile up my wallet, credit cards, and Jasjar on a table in my hallway. The magnets did a good job, admittedly :-)
The morale? Beware of hidden magnets in stuff that you put close to cards with a magnetic stripe. Your credit cards will say "thank you".