One scene involves graffiti that was censored by other people after it was written (see movie connections). Beneath the censoring, other people wrote things such as "There's no call for language like that" and "Agreed. Even if it is the apocalypse, we should be civil."
The Cold Stream dlc was a campaign that was never planned, it was conceived from a fan made campaign under the same name. This mod grew famous that Valve remade the map releasing it on Xbox and PC.
Chad L. Coleman also appears in The Walking Dead (2010), a live action show about survival in a zombie apocalypse.
The movie Zombieland (2009) was originally going to be based upon this game. However, the creators denied permission for the game to be mentioned in the movie.
"The Sacrifice" DLC was released for both L4D2 and the original game, although the DLC has proven to be difficult for L4D1 because its lack of L4D2's additional weapons and items. So, L4D2 is the recommended version for DLC purchase.