Getting through RFID key-card doors is a pain in the behind when your hands are full of books or files, and your key-card is in your pocket, or swinging free on a lanyard.
To address this problem, GeeDee86
let me have some redundant cards to play with, and I managed to turn the cards into bracelets. Caveat
: This technique should work on most cards with RFIDs embedded, but be aware that it is not acceptable to do it to some payment cards, such as the Oyster card you use to pay for your London commute. Check the ToS of your particular card, and look for clauses on mutilation.