Well that's its job surely? Yes it is, when it's working. Valve has issued an apology from Gabe Newell, and given 2 free copies of Left 4 Dead 2.
Now that's an apology. "This was our mistake, and I apologize for any frustration or angst it may have caused you," read to those players affected.
"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)."
Those unjustly banned are free to game once again. "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," added Valve's Newell.
Over 12,000 had been incorrectly banned by the automated system - quite the glaring mistake. It's great that Valve has put their hand up, and even better that they say they're really sorry with the gift of game - two in fact.
Maybe the Valve Anti-Cheat (VAC) system should screw-up more often?