Recently on Ars Technica they reported that Valve has admitted to accidentally banning over 12,000 CoD:MW2 players from the game due to an erroneous detection by the Steam software. According to Valve, the bans occured over a span of two weeks during which the disk image of the game was being updated.

Gabe Newell had this to say to effected users:

 

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.

 

To correct this blatant mistake on Valve’s part, they have granted every effected Steam account with a free copy of L4D2 as well as one copy to give to a friend as a gift.