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5 Cut Characters - VGFacts Five Trivia Feat. Caddicarus
 
The PC version of the game contains unused code that allows for split screen multiplayer. The code is functional, and it's possible to modify files to re-enable it.
Contributed by Stryker94
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There is an unused animation for the Hunter that shows him hanging upside down.
Contributed by PoisonTardis
In chapter 4 of the "No Mercy" campaign, in a room near the elevator, there are three x-ray images on the wall. One of them is of a hand that is missing the thumb: the Left 4 Dead logo.
Contributed by ISayWynaut
The achievement "Zombie Genocidest" is a reference to Dead Rising, in which the player must kill 53,594 zombies to receive the achievement "Zombie Genocider". In order to outdo Dead Rising, Valve included this achievement and added one more to the kill count. "Genocidest" functions as a play on words: going from the comparative suffix "er" (of Genocider) to the superlative suffix "est" emphasizes Valve's one-upmanship.
Contributed by ClaudX
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In chapter 2 of "The Passing" Campaign, there is graffiti on the wall referencing Dead Rising's Frank West. It reads: "Otis, out of film. No helicopters. Zombies too fast. Not going to make it.-Frank West"
Contributed by ClaudX
A number of infected that were visible in concept art were cut from the final game. These include:
• An infected with a large colony of rats bursting from its stomach, likely to be found in the sewer.
• A slow but strong giant called "Meat Wall" (which resembles a cross between a Boomer and a Tank).
• An infected named "Claws", which was possibly intended to be a male version of the Witch.
• An infected pig named "Hell Hog", likely intended for Blood Harvest.
Contributed by Megaman9000
In the Left 4 Dead (left4dead) and Left 4 Dead 2 (left4dead2 and left4dead2_dlc3) sound files, there are some sounds similiar to a dog. All of them seem to have been cut, but they still exist in the game files. The attack sounds for "zombiedog" were recycled for the Hunter when he'd attack with his claws. This could explain why the Hunter yelps like a dog when he is killed. According to some of the Church Guy's "combatcolor" sound files, the "dog" sounds seem to have been re-made for when he would turn into a Hunter.
Contributed by Funland47
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During development, there was another Special Infected called the Screamer, when attacked or agitated, the Screamer would run to a safe location and yell out a loud Scream, calling out a horde of zombies. While it was cut, many things were retained from it. The idea of attracting hordes was given to the Boomer in the form of his Bile. The concept of having to avoid an enemy completely or suffer the consequences was given to the Witch. His scream was given to the Hunter, and his maniacal cackle was given to the Jockey.
Contributed by Liraxus
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Originally, the Boomers bile was red, and he was meant to have no "puking" capabilities. Instead, he was to explode onto the survivors, dealing significant damage. This was later changed to incorporate the "Screamers" (a cut special infected type) horde-attracting abilities.
Contributed by Liraxus
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The game's cover was changed for the German and Japanese releases. The hand was changed to include the missing thumb and was made to look like it is tucked behind the hand.
Contributed by gamemaster1991
Bill's voice actor, Jim French, had a busy work schedule during development of Crash Course. As a result, Bill had no new lines and rarely spoke during the campaign.
Contributed by Antwan
According to the Left 4 Dead Commentary, the logic behind a rescue closet is that the "dead" survivor did not actually die, but was instead so badly hurt that they crawled into a closet, barricaded themselves in, and recovered somewhat, before noticing they couldn't get out.
Contributed by retrolinkx