In chapter 2 of The Passing, you can find a bar with a billiards room. Inside you can see some writing on the wall. One of these messages is written by "Frank West" informing "Otis" that he is out of film, and isn't going to make it out alive as the zombies are too fast. This is a reference to Dead Rising, which featured slow zombies, unlike Left 4 Dead's fast zombies.

When Left 4 Dead 2 was announced at E3 2009, fan outcry against the game developed into a petition led by two people. Valve responded by inviting the starters of the petition over to their headquarters to get a first-hand look at Left 4 Dead 2. Both founders were impressed and shut down the boycott one month later.

If you go up to any jukebox in the game, you have a random chance of getting one of five songs. One of these songs is "Still Alive" from Portal.

Several different versions of the box art for the game were made for different regions. The hand was flipped in the UK, as showing your 2 fingers in the manner shown on the US box art is considered an insult. For the Japanese and German versions, the hand is made to look like the thumb is tucked behind the hand instead of missing.

In the Dark Carnival campaign, you can get a garden gnome, named Gnome Chompski, as a prize for scoring over 750 points in the shooting gallery minigame. You must hold onto the gnome until the end of the level to get an achievement. This is a reference to Half-Life 2: Episode 2, in which you must launch a gnome into space in order to obtain an achievement.

A Special Infected called "The Leaker" was being developed. It had similar attacks as the Spitter and Boomer, and the ability to plant itself in the ground and act as a mine bomb. It was removed because the Survivors could run away from it, as the Leaker would alert the Survivors that it was planting itself.

However, it is still possible to play as The Leaker with console commands by using "Map MAPNAME Versus", then typing in console "Boomer_Leaker_Chance" and a number between 0 and 1. You will know when you are a leaker if it says "You are a/the Leaker!", or you explode on death and cause a small fire.

The Heavy Machine Gun was originally going to have limited ammo, but this limitation was given to the portable M60 machine gun instead.

The game was originally supposed to include a female DMV worker and a male firefighter but both were removed before the final version.

A riot shield was originally intended to be a usable melee weapon but was cut before the final version. Its fully textured model still exists in the game but can't be obtained without console commands or outside of custom campaigns.

Most of the graffiti in the bar references other zombie games and movies:

• Dead Rising - "Otis, out of film. No helicopters. Zombies too fast. Not going to make it. -Frank West"
• Plants vs. Zombies - "Need an M60? Come see me! -Crazy Dave"
• 28 Days Later - "Repent! The end is extremely fucking nigh."
• Shaun of the Dead - "Ed is king of the zombies."
• Dawn of the Dead (1978) - "Roger, went to the mall, Francine."
• Dawn of the Dead (2004) - "Went to save my dog. If not back in 15 minutes, drop everything and come save me. -Nicole"
• Venture Bros - "Hey MOL. Meet me in town we'll have some fun! -Brock"

In the commentary for Left 4 Dead 2, the developers stated that Nick's character was originally an escaped prisoner who stole a nice suit, but he was eventually adapted into his current con man persona. Nick is the only playable Survivor who retained his appearance from either beta screenshots or trailers out of all of the playable Survivors.

During development, The Park was conceived as a procedurally generated maze. However, in testing, this was found to be too distracting from gameplay and difficult for players who often got lost in the area. However, the concept was salvaged in the The Cemetery wherein The Director chooses from four alternative layouts and navigation pathways.

DLC based on the horror movie The Cabin in the Woods was once in development. However, when the movie studio MGM filed for bankruptcy in 2010, the content was delayed and ultimately scrapped.

Due to censorship laws in Germany and Australia, some weapons were changed to Counter Strike: Source weapons instead. All of the weapons can be accessed using console commands in the other localized releases, with the exclusion of the Combat Knife, which can only be accessed in the German and Australian releases.

The Chainsaw can kill every infected (except the Tank) in 3 seconds.

In early concept art, there is a picture of Coach using a compound bow, which is not present in the final game.

Due to the amount of witches in Hard Rain, it can be inferred that witches are drawn to sugar. Players may expect 3 to 5 witches in The Sugar Mill level alone, but up to 20 can spawn normally, often in areas where you can't avoid them.

In Blood Harvest and Swamp Fever, several cows can be found with the skin on there heads shaved off. The infection in the game, Green Flu, is thought to be a mutated rabies virus in game, and spread through livestock. It is possible that these cows are the result of the local people performing rabies tests on the cattle to see if they are infected.

The M60 machine gun uses the same reload animation as the AK47, however it can't be seen during regular gameplay due to the M60's limited ammo.