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.

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.

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.

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.

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.

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.

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.

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"

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.

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.

There is an unused animation for the Hunter that shows him hanging upside down.

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.

According to the Co-creator of Left 4 Dead, Chet Faliszek. The original two conceived titles for Left 4 Dead were: "Terror Terror Terror!" And "Blood Run".

"Terror Terror Terror!" had briefly been trademarked before being dropped as a title, but "Blood Run" was rejected promptly due to Faliszek hating the word 'Blood' being in it.

Eventually after a list of titles were suggested, Erik Wolpaw was the one to come up with "Left 4 Dead".

According to developer interviews, Left 4 Dead was originally conceived as a multiplayer game where a team of player-controlled survivors would fight against a team of player-controlled infected. It is unknown how much work was done with this version of the game, or even if a playable build even exists. While not the main focus in the final game, a similar mode would exist with the 'Versus Mode'.

Every main survivor design in the original beta of Left 4 Dead was drastically different to what was in the final game, originally using stock photos of models from the 3D.SK website of references to model them after. This is also what the textures of the zombies are based on in the final game.

Once Valve stepped in, they decided to redesign the survivors, because they thought they looked too well-prepared for the apocalypse, which within the game's lore had just begun. They also believed that if the survivors were in a more casual and everyday design, then they would resonate with a wider audience.

Valve also wanted to have cinematic cutscenes in the final game, and thought that the designs were too simplistic and wouldn't translate well into them. So instead of using stock photos for their new designs, they instead hired actors to be directly modeled for the survivors to give them a higher quality design: Bernard Fouquet (Bill), Sonja Kinski (Zoey), Sean Bennett (Louis), & Taylor Knox (Francis).

Out of the 4 main survivors, Zoey was the only one to change voice actors, changing from Alésia Glidewell to Jen Taylor. Despite this, almost all of the pain and death sounds you hear in the game are actually lines leftover from Alésia and not Jen. It's unknown if this was intentional as she is not credited in Left 4 Dead.