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Doom - Did You Know Gaming? Feat. Markiplier
 
The first level in Doom was actually created last. This is because John Romero wanted experience in level designing so he could make the first level his best.
Contributed by TheGrayGamer
The idea to use demons as enemies came from a Dungeons & Dragons campaign played by some of the developers, in which the game ended with demons spawning from a portal and overflowing the world.
Contributed by skitsjazz
Doom was so addictive upon its release that businesses and college campuses all over the world noticed a massive drop in productivity. Students and employees would use college and company time to play multiplayer, which would cause network overloads. Even John Romero himself was incredibly addicted and would lock himself up in his office so he could play Doom with his baby. This led John Carmack with no choice but to break down his office door with an ax to get him to stop playing.
Contributed by raidramon0
Episode 1, Mission 4 (Command Control) changed between different versions of the game. In the initial release version (1.0), the map contained a Swastika-shaped arrangement of computers, in a reference to Wolfenstein 3D, an earlier id Software game. This was later changed in version 1.4, the "final" version.
Contributed by TankCatapult
In the PlayStation 1, 3D0, Atari Jaguar and Sega Saturn versions of the game, the "I'm Too Young to Die" skill level is replaced with "I'm A Wimp".
Contributed by TankCatapult
There's an unused EA logo found in the 3DO port. This was from a deal with EA to distribute the game which ultimately fell through.
Contributed by KnowledgeBase
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The 3DO port of the game, which is widely considered to be one of the worst, had suffered due to the limitations of the system and misinformation from the publisher. According to Rebecca Heineman, who had worked on the port, the game's publisher, Art Data Interactive, told her before she agreed to work on it that the version already had new levels, weapons and features and that it only needed to be 'polished'. This was because they thought that to port the game, it was simple enough to take it from one platform to the other and compile the code, whilst adding weapons in it would be as simple as inserting artwork.

Heineman had to begin from scratch starting in August 1995 with a target to complete it by October. With no time to port the original music, she had to record new tracks with a band hired by the publisher. Heineman even had to write specific apps for the 3DO and Logicware in-game logos then delete them from the memory because the console couldn't handle logos. The floors and ceilings in the game were drawn with software rendering due to the 3D limitations of the platform.

Heineman released the source code for the port in 2014 on the website GitHub.
Contributed by KnowledgeBase
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In the Doom Press-Release beta, dated October, 1993, the BFG9000 had a very different firing sprite and animation. The animation consisted of multiple red and green plasma orbs firing. This was later changed to reduce strain on computers at the time.
Contributed by TankCatapult
Despite always going by the name "Doomguy" or just "The Marine" since he was never named in the games, the protagonist got a name in the novelization of the series as Flynn "Fly" Taggart.
Contributed by Mass Distraction
On the Sega 32x version of the game, cheating or using the level select option will cause the game to end after level 15, displaying the credits and a fake DOS prompt, rather than proceeding to the next level.
Contributed by RetroRex
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In the map "E4M1: Hell Beneath" in the "The Ultimate Doom" release, there is a room that features the logo for the band "Nine Inch Nails".
Contributed by LordyLord
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In versions of Doom prior to 1.4, a room in the level "E1M4: Command Control" features a sector shaped like a swastika. According to John Romero, this is a reference to Wolfenstein 3D, an FPS previously developed by id where the protagonist fights Nazis.

"Yes, [in Commander Keen 5] there is a swastika in one of the levels, one of my levels to be exact, but I removed it shortly after the game was released because people were upset that an evil symbol was in a cute kid's game. It was a premonition of things to come, namely, Wolfenstein 3D. I also put a swastika in Doom's E1M4 as a Wolf3D reference, but I changed it later for the exact same reason."
Contributed by LordyLord
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The space marine makes many faces in the game depending on the player's status. One of these faces is a "shocked" face known colloquially amongst Doom fans as the "Ouch Face", which rarely makes an appearance due to a coding error in which the programmers used "(plyr->health - st_oldhealth > ST_MUCHPAIN)" instead of the correct expression "(st_oldhealth - plyr->health > ST_MUCHPAIN)". While this face was meant to show up if the player took 20 or more hit points worth of damage in a single tic, the error meant that it only displayed when the player "gained" 20 hit points while taking damage. An example of this can be seen when attempting to use the IDDQD cheat during the very end of E1M8: Phobos Anomaly.

A number of source ports fix this behaviour, allowing the Ouch face to display as originally intended.
Contributed by TheW2A_GV
The Cyberdemon lost his arm and leg during a fight against BJ Blazskovicz, the hero from the Wolfenstein series, during the events of Wolfenstein RPG.
Contributed by pAroxizm
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Cacodemon's general visual design is similar to that of the Beholder, a similarly one-eyed classic Dungeons & Dragons monster (with eye stalks instead of horns). In addition, the Cacodemon was created from a cropping of a creature that appears on the cover of Manual of the Planes, a Dungeons & Dragons expansion book. The creature itself is known as an Astral Dreadnought, and was created by Jeff Easley for that book.
Contributed by Funland47
According to John Carmack, the name Doom comes from a line in the movie 'The Color of Money'. In the movie, Vincent Lauria (played by Tom Cruse) shows up at a pool hall with a custom pool cue in a case. "What do you have in there?" someone asks. Vincent replies "Doom." with a cocky grin.
Contributed by gamemaster1991
In 1995, it was estimated that Doom was installed on to more computers than Microsoft's own Windows 95. This led Bill Gates to briefly consider buying id Software, so they could have the rights to the Doom name. This also led to the development of a Windows 95 port of the game to promote the operating system as a gaming platform.
Contributed by gamemaster1991
The title of the fourth episode of Ultimate DOOM "Thy Flesh Consumed" as well as the level names are derived from the King James Bible.
Contributed by MightyKombat
Doom has actually been made to run on graphing calculators, though it only runs for a short time before the batteries are completely drained.
Contributed by Shadowmane
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The "NIGHTMARE!" difficulty didn't exist in the initial versions of the game. It was added in version 1.2 after players jokingly said that the "Ultra-violence" difficulty wasn't hard enough.
Contributed by ThisIsGamer
Several songs from both Doom and Doom II: Hell on Earth are based on heavy metal songs from the 1980's and early 1990's.
Contributed by ThisIsGamer