A warning bulletin is a list of credit cards that are reported stolen, canceled or compromised in some way.
For example, let's assume that John's wallet is stolen. He calls Visa to report the theft, and the company places his number on its warning bulletin -- an electronic list maintained by Visa.
Later that day, a $5,000 transaction at Nordstrom appears on the credit card. The sales associate retains the card because the point-of-sale (POS) terminal quickly matches the card number to the number Visa placed on its warning bulletin. The sales associate then follows company policy regarding such situations -- this might include retaining the card and calling the police.
Credit card fraud costs businesses and individuals billions of dollars a year -- at least $8.6 billion, according to one study. Due to the sheer number of credit cards out there and the massive number of transactions that occur every day, credit card processors need a way to communicate lists of lost, stolen or compromised card numbers quickly and efficiently. The warning bulletin is one such method.