A new Counter-Strike update contains an extremely early prototype of zombie co-op shooter Left 4 Dead. To celebrate the 25th anniversary of Half-Life, Valve has released a variety of new material for its epochal, single-player FPS. The developer has also updated the legendary Counter-Strike 1.6, in order to make it compatible with changes in Half-Life itself. However, in the process of updating CS 1.6, it appears Valve has accidentally included a prototype of L4D that was built as a mod for Counter-Strike Condition Zero. If you want a change from Counter-Strike 2, here’s something very different indeed.

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