Over on Reddit, user rofulz notices a striking similarity between a vaguely industrial looking warehouse in CS:GO and a vaguely industrial looking, decrepit warehouse in Left 4 Dead 2. If it had been any other developer, we'd cry foul over lazy re-use of assets. But as this is Valve we're talking about, there's only one real explanation. Counter-Strike: Global Offensive is a Left 4 Dead prequel.
Here are both locations, as shot by the keen-eyed rofulz.
That's close enough to be a deliberate reference, yet different enough to not have been a blind copy and paste job. After all, Valve have a history with plugging their universes into one another: with Half-Life and Portal widely accepted to exist within the same world. The most popular crossover theory in this case suggests that the counter-terrorists of of CS:GO are fighting to defuse the biological weapons that spark off the infection of the Left 4 Dead games.
That said, plot is so thin on the ground in both that you can pretty much draw any conclusion you like. One redditor points out that Left 4 Dead's Louis actually mentions Counter-Strike in his dialogue ("man, this is just like Counter-Strike!"), which would threaten the theory somewhat, had another canny redditor not insisted: "There can be a game called Counter-Strike in the Counter-Strike universe. Marvel Comics exists in the Marvel Comics universe."
Further examples of Left 4 Dead maps finding their way into CS:GO, with jazzy synth-funk accompaniment.