A Japanese vendor called Shelly sells Hello Kitty (and other characters) RFID-card sleeves that shield them from “skimmers” who read the card and copy its contents, so that later they can spend your money, break into your car or home, ride the subway as you, etc.
Of course, this negates much of the value of RFID cards: no more can you merely wave your handbag at a turnstile. Now you have to get your card out, remove the radio-proof sleeve, and then rub the naked card over the reader.
And at that moment, if a skimmer happens to have a directional antenna pointing at the turnstile, well, then she can read (and clone) your card.
(via Beyond the Beyond)