Well, Left 4 Dead was fun while it lasted. But it appears that you're going to have to deal with some serious douchebaggery at some point during one of your versus mode romps.
Reader Mario wrote to us with a distressing tale, saying "Versus mode is a blast, but recently, I joined a game and found myself in a safehouse at the beginning of a chapter. Suddenly, about 30 witches spawned in the safehouse with me followed by two tanks." How? Console command-style hackery that can be used on the Xbox 360 version. Cute when it happens to the next guy, but probably not to you.
You can see it in action in the clip above, which will also unfortunately tell you how to do it. We're contacting Valve to see if it's taking action to address Left 4 Dead cheating.