Valve didn’t just update Half-Life for the game’s recent 25th anniversary. It also spruced up Counter-Strike 1.6 to take advantage of the features and changes added to Valve’s 1998 masterpiece. Yet in doing so, Valve seemingly inadvertently opened the door to a fascinating slice of gaming history – the first prototype of Left 4 Dead.

This prototype, dubbed “Terror Strike” was built for Counter-Strike: Condition Zero, and it’s essentially a proof-of-concept for Left4Dead’s core gameplay using CS assets as placeholders. It sees the player exploring a white-boxed map of an urban area, using Counter-Strike weapons to fight off AI Counter-Terrorists coded to swarm the player like zombies.

The prototype was revealed by Valve content creator Gabe Follower, who posted a video of Terror Strike on the website formerly known as Twitter. “Valve accidentally compiled and pushed earliest Left 4 Dead prototype called “Terror Strike” into the latest CS 1.6 update and [the] community figured out how to play it.”

According to Gabe Follower, a user going by the name “The One Epicplayer” (classic) ported the map and “did the bot navigation mesh”. The goal Terror Strike is for the player, assuming the role of a terrorist to get to a certain building in the map and plant the bomb while under attack from the “zombies”. So far, so Counter-Strike. Once the bomb is planet, all the dead CT zombies respawn and try to swarm the player, who have to kill all the zombies while the bomb is planted.

Curiously enough, this is the second Left 4 Dead prototype to leak this year. Way back in January, another version of Terror Strike appeared online as a playable mod. This is functionally identical to the more recent leak, but instead being made for Condition Zero, it’s based off Counter Strike: Source. This gives us a pleasingly granular view of the evolution of Left4Dead, first in grainy GoldSrc goodness, before moving into Valve’s shinier Source engine that would ultimately form Left 4 Dead’s technological basis.

Of course, it’s entirely possible that Valve didn’t do this by mistake. The company also appeared to have fixed a bug in Half-Life based off its appearance in the game’s own 25th anniversary documentary, but it turns out fixing the bug was the plan all along, Valve just ran out of time to do it before the special occasion.