Most cards are magnetic strip swipe cards, and can become demagnetized. The most common reason for a room key to get demagnetized is the use of cell phones. I have seen many a guest carrying their cards in the same hand as their cell phones. Receive a call and it will be demagnetized.

In 2026, magnetic stripe hotel cards remain highly susceptible to demagnetization, though many hotels are transitioning to RFID "tap" cards which are immune to this issue. Traditional cards store data on a magnetic strip that can be easily "scrambled" if it comes into contact with strong magnets. The most common culprit is the magnetized clasp on a smartphone case or a magnetic money clip. While the internal magnets of a smartphone itself (like those used for MagSafe or speakers) are generally not strong enough to erase a card instantly, prolonged contact—such as keeping your phone and key card in the same pocket for several hours—can degrade the data enough to make the card unreadable. High heat, such as leaving a card on a car dashboard in the sun, can also warp the magnetic layer. To protect your card, it is best to keep it in a separate wallet slot away from electronic devices and magnetic accessories. If your card fails, the front desk can usually "re-magnetize" or re-encode it in seconds at no charge.