It’s an old saying that is often used by hotel staff when issuing door lock swipe cards. “Don’t place your card near other cards or your mobile phone. It may get wiped and will no longer work.” But is it true, or a myth? If it is true there is a whole industry of Phone Wallets in question.
There seems to be little evidence that mobile phones will wipe mag stripe cards however it is often the first line trotted out by the concierge when you return a faulty hotel door card. In my 18+ years in the Plastic Card Industry I am yet to have first hand experience of a mobile phone wiping a mag card.
There is some conjecture that poorly encoded cards on cheap mag stripe encoding equipment may corrupt when brought near electrical fields (including an old style TV). This may have been where the rumor had its roots. These days its more likely a faulty card or the card encoder behind the desk needs cleaning and it’s a convenient story to deflect blame.