Left 4 Dead on the Xbox 360 had been over-run by hackers recently, prompting Valve to quickly fix the servers to stop them ruining the game.
This video shows what the hackers have been getting up to - creating giant witches, spawning weapons everywhere, infinite pipe-bombs, spawning AI and so on.
One player reports of starting a game and having 30 witches and two tanks attack him in the safe-room, when you usually only get one per level.
Valve has fixed this online and will release an update to prevent it happening on system link too. The current fix is something they applied on their servers, so you don't have to download a patch to prevent these cheats from happening.