Attentive horror fans may have spotted the bilious Boomer and other Left 4 Dead monsters snuck into Joss Whedon's delightful scare 'em up movie The Cabin in the Woods, but L4D's involvement was once supposed to be far greater. Director Drew Goddard has revealed that a CitW expansion was planned for Left 4 Dead 2, but sadly got scrapped. Plainly, this post contains spoilers for the film, which you really should see if you haven't.
"We actually were going to do a downloadable L4D2 expansion pack, where you'd fight in the Cabin world, but then MGM went bankrupt so the delay squashed it," Goddard said in an 'Ask Me Anything' on Reddit. "But the people at Valve were still cool enough to let us use some of their monsters to fill the cubes in the background (I had a lot of cubes to fill.)"
He added, "By the way -- the game was gonna be amazing. You were gonna be able to play in both the upstairs 'Cabin in the Woods' world and the downstairs 'facility' world with all the monsters. Believe me, I HATE all video games based on movies, they always suck, but porting Cabin into Left For Dead felt like the right fit. It pains me that it didn't happen."
Here's the scene where the Left 4 Dead monsties appear: