Today, someone phoned Citibank's toll-free number (800-274-6660) and claimed to have found my credit-card, which meant that it was immediately and irrevocably cancelled. Citibank didn't phone me to find out whether the card was in my possession, nor did they require the security code printed on the signature strip — all they needed was the number. Add this to the list of things you can do with someone's credit-card number: cancel it.