1,341 Steam keys have been disabled following a shady deal involving what looks like stolen credit card information. Natural Selection 2 devs Unknown Worlds took the step after it became apparent that the keys were purchased through unofficial third-party sites.
The codes were initially bought using what is likely stolen credit card information, and then sold on via third-party sites. When the individual who had unknowingly purchased the codes realised what had happened they initiated a charge back - a security measure that allows people to claim back disputed money in cases like this.
Ultimately that left Unknown Worlds to foot the bill, and so they requested Steam disable the tainted codes. In a post on their forum the devs explained:
We deactivated these keys because they keys were purchased with credit cards where the card-holder initiated a "charge-back." A charge-back is a consumer protection mechanism offered by payment companies such as Visa, allowing a card-holder to dispute a charge on their credit card statement. This means we never received payment for the game. In fact, we were charged a fee by the card issuer for the charge-back. For these 1,341 keys, these fees totalled around $30,000.
We don't know how exactly these sites obtain their Steam keys. It seems likely that they were originally obtained from our store using stolen credit card information. Keys were then sold through a handful of questionable sites to people using legitimate credit cards. The owner of the stolen credit card ultimately disputed the charge and we lost the sale. In total, we lose ~$45 per transaction of this kind, due to the charge-back fee (~$22 fee + $25 game price). Meanwhile, the unauthorized key reseller kept the money from the player who ultimately received the bad key.
If your key was deactivated, we recommend you contact the site you purchased it from for a full refund. It's unfortunate that players who believed they were buying legitimate games were hurt by these unscrupulous resellers.