10 Things Video Games REALLY Didn't Want You To See

10. A Left 4 Dead Prototype - Counter-Strike: Condition Zero

The 1.6 update for Counter-Strike: Condition Zero released last November also added a most peculiar feature to the legendary FPS - a very early prototype of Valve's 2008 zombie-themed FPS Left 4 Dead.

The prototype, called Terror Strike, was accidentally compiled into Counter-Strike by Valve during the 1.6 update, allowing players to experience an extremely rudimentary version of Left 4 Dead's gameplay, functioning as a mod for Counter-Strike.

Using Counter-Strike assets, players must fend off a horde of AI foes programmed to attack them with the rabid ferocity of, you guessed it, zombies.

Valve didn't take long to remove Terror Strike from the game, but by this point slippery-fingered fans had already copied it and created their own unofficial Counter-Strike mod, effectively allowing it to be preserved in perpetuity.

Even though Valve evidently didn't want this early slice of Left 4 Dead's development to be made public, it's now a piece of public gaming industry they can't ever hush up, so they're better off just embracing it.