In the movie The Cabin in the Woods, in the scene with the cubes containing many monsters, you can see a Witch, a Tank, and a few seconds later, a Boomer. This is the remnant of a bigger deal between MGM and Valve, whom were originally going to release an entire DLC pack for Left 4 Dead 2 based on The Cabin in the Woods. MGM however went bankrupt, and the deal fell through, these three infected were permitted to be used under Valves courtesy.