Sandy, by the way, is a Mormon. Which didn't stop him from helping to develop a game crammed with Satanic elements. Quoth the man himself to John Romero:
"I have no problems with the demons in the game. They're just cartoons. And, anyway, they're the bad guys."
One of the secret levels in the PlayStation port of the original games is a nightclub, complete with rave music.
The shotgun weapon sprites from the classic games are scanned images of a Tootsie Toy cap gun.
The plasma gun sprites, similarly, are scanned images of the front half of an M60-esque Nerf gun turned backwards.
One step backwards from the engine used in Wolfenstein 3D is that sectors could not move horizontally. A result of this is that Doom doors, at least in stock maps, always open upwards, compared to the sideways-opening doors and pushwalls to secret areas in Wolf3D — it was not possible to replicate that sort of behavior on the Doom engine until the version that Hexen was built on introduced polyobjects.
Ascended Fanon: Out of all the nicknames and made up names, Doomguy seems to be the one which the creators have at least accepted as a Half-Canon Name (if largely for simplicity), as it got numerous nods and actually appeared as the Marine's name in THPS3.
Fandom Life Cycle: The classic Doom games seem to be in stage 6a, with an active community.
Fan Nickname: For a long time, the protagonist of the original series never had an actual name and was referred to as "Doomguy" by the fans. It was not until the release of the novels that he was finally given a real name: Flynn Taggart.
Not Safe for Work: Doom is notably deemed Not Safe for Productivity. Its wide distribution made it one of the most popular leisure applications to be found at workplaces and universities. One quit message in Doom II parodies it.
The BFG Edition includes revised versions of the classic games; among the changes are the replacement of the red crosses on medical supplies with a red-and-white pill icon. This is in accordance with the American Red Cross' wishes for its symbol to not be used in further video games.
Of the textures created from scanned pictures, one was based on one artist's snakeskin boots, while another was based on a photograph of another artist's wounded knee.
Port Overdosed: There has scarcely been a platform since Doom's release that has not received a version. The game is famous for its utter ubiquity.
The team at Microsoft who created the Doom95 port for the Windows OS was headed by Gabe Newell.
Stock Sound Effects: The grunts and moans made by former human enemies are just some the noises made by the common dromedary camel.
Urban Legend of Zelda: Shortly after the Columbine school shooting, it was discovered that both boys were fans of Doom, which led to claims that Dylan Klebold made custom WADs based off of the school for "training". While Klebold did make levels (including a set of levels with graphics that made the game Bloodier and Gorier), none of them are modeled off of Columbine High, and when taken on their own reviewers have called the "Klebold Levels" average to sub-par.
What Could Have Been: Tom Hall, one of the lead developers, had plans to wove in a more detailed story for the game and introduce several fleshed-out characters. John Carmack was heavily against this idea, claiming that doing so would overcomplicate the game, believing that the concept of "Shooting demons on Mars" would be enough to keep the players invested. It was, but the creative differences led to Hall resigning from the company before the game was finished.
Several weapons planned for the first game included a 'Dark Claw' (which would've drained life from enemies), 'Probjectile' (same as the regular pistol except it gives you enemy stats), 'Spray Rifle' (same as the shotgun, but wider shots), a bayonet (another melee weapon, attached to the rifle that would have been the player's default weapon) and the 'Unmaker' (a weapon that would've done more damage depending on the enemy types).
Doom II: Hell on Earth
The music for the ninth level, "The Pit," originally featured a piano as the main instrument. It was rejected because the rest of the development team wasn't keen on having piano music in Doom. Following this, the composer changed the piano to a harpsichord as a joke. The staff didn't notice it was a joke and accepted it.
John Romero's famous head on a stick was a joke on Romero by the artists, as the graphic used for the final boss's weak point was originally nothing out of the ordinary. His response was to record the equally famous backwards message that's heard when you face the final boss. Which, of course, is why it says, "To win the game, you must kill me, John Romero!"
One of the rock textures is actually from Wolfenstein 3D. And we don't mean in the secret Wolfenstein levels, either.
The TNT in TNT: Evilution actually stands for "The New Technology."
TNT also derives from the creators, Team TNT, who were a modding group for Doom; the commercial addition of Evilution to Final Doom was rather detested by the Doom community at the time, as TNT: Evilution was assumed to be another free megawad, right up until the day of its intended free release. This is an early example of a third-party mod being adopted by the original creators.
Dario and Milo Casali, the creators of The Plutonia Experiment, contributed four levels each to TNT: Evilution when it was a freeware Game Mod, only to have most of them rejected by id Software during its commercialization for being too resource-demanding for most contemporary computers. At least a few of them were reused in Plutonia.
Keep Circulating the Tapes: Of all the Doom games that have been ported and/or re-released on various consoles over the years, Doom 64 remains to be the only game that has yet to leave the platform it originated from. Well, not officially that is; there is a fan-made total conversion named Doom 64: Absolution that re-creates Doom 64 on PC with the advantages a PC game would offer. The project was succeeded by Doom 64 EX, a Fan Remake by the same creator which aims to be a much more faithful re-creation of the game than the original total conversion with some new added features. Second-hand copies of the game are circulating through online retailers and auction sites.
Working Title: The original name of the game was actually titled The Absolution during its development. The reason for changing the original title with the Doom 64 moniker was for brand recognition, however the working title was used as the name for MAP28. Unfortunately, this lead to many people to assume the game would be just another port of Doom rather than an entirely new chapter in the classic series.
Theresa Chasar, the female UAC scientist who becomes the first Lost Soul, was named after the sister of Tim Willits, the lead designer.
Think the Soul Cube the Artifact were unique weapons to Doom 3? Think again. The original Doom had them first, in the form of the Dark Claw and the Unmaker. Both of these weapons were only seen in early alphas of the game and never made it into the final product, however the Unmaker later appeared in Doom 64. These weapons were originally supposed to work similarly to the Soul Cube and the Artifact; the Unmaker even has "upgrades" to it, in the same manner the Artifact gains its powers.
Hey, It's That Voice!: In Resurrection of Evil one of the audio logs is spoke by Scott Menville, who did the voice of Robin in the Teen Titans cartoon. Hearing the voice of Robin speaking in utter terror about demons is pretty funny.
Knights of the Old Republic veterans may recognize Councilor Elliot Swann (played by Charles Dennis) as Davik Kang (the first game) or Lt. Dol Grenn (the second one). He also voiced Zherron in the latter game, but he's more difficult to recognize in that role.
A good majority of background characters and audio logs are voiced by SpikeSpiegel.