Wuh-oh, for some reason, an “extremely early” Left 4 Dead prototype has been found thanks to a recent Counter-Strike: CZ update.

Video games are weird little things – sometimes a dev will build something in a game that ends up unused, and it just has to get locked away as removing it could risk breaking everything else. That sometimes leads to fun little discoveries, and as shared by PCGamer, in Counter-Strike: CZ’s recent 1.6 update, a Left 4 Dead prototype titled “Terror Strike” was discovered and is now completely playable. Valve content creator Gabe Follower shared word of the prototype on Twitter, pointing towards a user “The One Epicplayer” who apparently ported the zombie_city map as well as the bot navigation mesh.

According to YouTuber ilovethevopo, “what you see is gameplay of Left 4 Dead back when it was a simple project using Counter-Strike: Condition Zero. Yes, this is older than zombie_city and the leaked L4D footage that was re-discovered a few weeks ago, both of which were based on Counter-Strike: Source. Zombie_city was originally built for CZ and ported to CSS, but we never got any actual content from the CZ era of L4D until now.

“The code was removed, but not before fans snagged 1.6’s dll files and worked some magic to make a quick ‘mod’ built on Condition Zero. The Terror-Strike code was included because 1.6 has code from CZ in it, and it seems someone forgot to strip the Terror-Strike code from the dlls before originally shipping the update.”

While its removal obviously indicates that this is something Valve doesn’t want us seeing, it’s still a cool bit of video game history, as it’s rare we get to see such early prototypes of games. Obviously in this particular case it looks quite simple, but it did go on to spawn Left 4 Dead 2 after all.

Counter-Strike 2 finally launched back in September, though it did leave a number of players mourning lost content as the original game is no longer playable.