Valve Software is offering two free copies of its game Left 4 Dead 2 to the 12,000 players it erroneously banned from playing Call Of Duty: Modern Warfare 2 via Steam.

A bug in Valve's digital distribution platform caused the company's proprietary VAC anti-cheat tech to believe thousands of Modern Warfare 2 players had hacked their games.

Instant bans from playing the Call of Duty title followed, which resulted in an outpouring of forum posts and messages to gaming sites. Valve support staff had reportedly told players that they were unable to help.

Valve has now admitted that the error was its fault, and as well as lifting the bans has gifted the accounts of those affected with two complimentary copies of Left 4 Dead 2.

The company's boss Gabe Newell last night sent out an apologetic message to the accounts concerned:

"This was our mistake, and I apologize for any frustration or angst it may have caused you," he said.

"The problem was that Steam would fail a signature check between the disk version of a DLL and a latent memory version. This was caused by a combination of conditions occurring while Steam was updating the disk image of a game.

"This wasn't a game-specific mistake. Steam allows us to manage and reverse these erroneous bans (about 12,000 erroneous bans over two weeks).

"We have reversed the ban, restoring your access to the game. In addition, we have given you a free copy of Left 4 Dead 2 to give as a gift on Steam, plus a free copy for yourself if you didn't already own the game."

The outrage does not appear to have greatly affected the Activision-published title's fortunes on Steam, however. Modern Warfare 2 remains the third-best seller on the service, with its DLC the Resurgence Pack taking the top-spot.