Alright, here we go.
- As cosmic horror John Carmack revealed in a 2017 interview, when Doom 2 was in development, iD Software was testing support for a peripheral that would shoot the player in real life when they got hurt ingame. This was scrapped when they found out that shooting playtesters would make them unable to play the game.
- True freelook without Y-shearing is possible in the original Doom engine, but was disabled on purpose as the sky textures were accidentally made too short.
- Doom was originally going to be called Quake, as proposed by Tom Hall. However, Tim Willits (known as the inventor of 3d games) said Quake was a stupid name that should never be used for anything, prompting everyone else at iD to kick Tom Hall out of the group.
- The critically acclaimed crate maze in E2M2 was inspired by a dream Tom Hall had where he got lost in a warehouse. After Tom left and Sandy Petersen was brought in, he placed a berserk pack within the level so he could punch the crates as much as he wanted. This accidentally led to what is often called the best level in Doom.
- The myth that CDs (Carmack Discs) were invented specifically to store Doom 2 in a more compact way is incorrect. They were invented as an attempt at a more efficient pizza cutter technology involving red lasers, but it was discovered by John Romero that the discs worked great as a data storage format, given some tweaks.
- The lovable protagonist of the Doom series, John Doom, was inspired by a random guy John Romero allegedly met at a pub. Efforts made by Doomworld forum users to locate this person have showed no results.
- John Carmack was initially dead set on including molecular-level liquid physics simulation in Doom, however it was cut for time, leading to floors being disguised as liquids in the final release.
- John Doom and the Quake III: Arena character "Phobos" are canonically husbands. This is a well-known fact, and I will not indulge any arguments.
- Bobby Prince, being a lawyer, knows every law in existence including copyright law. This is why he officially licensed every music track found in Doom 1 & 2.
- The Mancubus in Doom 2 was modelled as a realistic depiction of your mom.
- After the extreme popularity of Doom 2016, a real-life full scale replica of the fictional planet Mars was commissioned by Bethesda and launched into space as a promotional technique. It stays there to this day.
- As a devoted Christian, Adrian Carmack hated the demonic themes of Doom. Longing for the good old days of iD, he put a secret Commander Keen reference inside Doom 2.
- The myth that the Doom engine isn't true 3D has long been disproven. Doom is in fact a 5D game.