Backdoored
A "backdoored" card is one that has been stolen from the manufacturer, or "taken out the backdoor" so to speak. This often happened by card company employees who had access to the cards at the factory, but after many complaints card companies began to crackdown on the internal theft, but from time to time we still hear about these kinds of incidents.
It is standard procedure for trading card companies to make additional copies of each card (even those with serial-numbers) to use as replacements for cards that get damaged during the shipping process and at the stores where packs are sold. The companies do this so they can satisfy an unhappy collector who might find a valuable card that is damaged. This way the company can send the collector a nice replacement for their damaged card. However, what happens to these cards if they are not used as replacements? That much is still a mystery.