The failure rate of KTTW cards is pretty high. Because of this, people constantly ask, "Why is this happening? How can I stop it?" And CMs, trying to be helpful, continue to pass on info that's incomplete or inaccurate.
Failure of the magnetic strips on KTTW cards are caused by several sources - which is why so many people are flabbergasted when their cards fail even though they keep them religiously away from any electrical device.
Failures are caused by three major sources:
1) Physical damage to the strip.
Damage can be obvious rips in the magnetic strip, or cracks in the plastic card, but more often the damage takes the form of very small scratches from passing the card through turnstiles, FP machines, or cash registers, or from storing the card in a pants pocket, wallet, purse, backpack, fanny pack, etc. These scratches cannot allways be seen by the naked eye, since they often run parralell with the strip itself and are often very small - but they can and will cause the magnetic strip to fail.
2) Static electricity.
Static happens everywhere, especially in cloth pockets. Put your KTTW card, bare, into a pocket, and not only can it get scratched by the cloth and the other items in the pocket, but any static charge generated by the rubbing cloth as you walk can scramble the strip.
But even if you keep your card in a protective pouch or sleeve, static from your clothing can discharge when you ground yourself, and sometimes it can pass through the card and scramble the strip.
3) Strong magnetic fields.
While this one is the most-cited reason for card failure, I personally think that it's the least common of the Big Three. To cause failure of a magnetic strip on a credit card or KTTW card, a magnetic field needs to be A) strong, B) close, and C) in motion across or along the strip.
CMs often cite cell phones, magnetic purse or wallet clasps, or even the mythical "electric eel-skin wallet" as the culprit for a KTTW card failure, but after seeing the difficulty that the Mythbusters had when trying to cause a malfunction in a credit card strip with magnets - it wouldn't fail till they used a really POWERFULL magnet - I tend to doubt that the tiny magnets in most wallets and purses, or the almost-imperceptible magnetic field from a cell phone, FRS radio, or camera battery, could do any appreciable damage to a credit card.
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KTTW cards do fail at a much higher rate than Tyvec MYW tickets, however, so my guess is that the magnetic media used on KTTW cards is much more sensitive and fragile than the media used on Tyvek tickets. As such, it's probably a lot more sensitive and fragile to the magnetic media used on credit and debit cards, so prudence demands that we excercise caution with all WDW ticket media, either plastic or Tyvec.
In the years since WDW started using magnetic strip cards for tickets and room keys, I have never had a KTTW card or a Tyvec ticket fail. I had a room key from an off-site hotel fail once. I keep my tickets and room keys in a vinyl pouch on a neck lanyard, only take them out when they are needed, and handle them by the edges when they are out of the pouch to avoid damaging the magnetic strip. This seems to work for me.