Did you know that several monsters make characteristic sounds while roaming?
Did you know that the Nightmare! skill level was added as a joke, in response to comments that the game was too easy?
Did you know that many sound effects used in the original Doom games were acquired from Sound Ideas' General series sound effects library, which is why they can be heard in various movies and television shows?
Did you know that arch-vile can, eventually, kill themselves from the splash damage from their own attack?
Did you know that the abbreviation BFG stands for "Big Fucking Gun"?
Did you know that splash damage in the original Doom games is computed only from the horizontal distance between the explosion and the affected thing, which can be used to harm targets far above or below the blast?
Did you know that in the original Doom games the speed of projectiles does not take the vertical dimension into account? For example, this causes monsters' fireballs to travel extremely fast when fired from above or below.
Did you know that the Strife ID badge and ID card have switched around their lock messages?
Did you know that the "idchoppers" cheat code is meant to make the player invulnerable in addition to giving him a chainsaw, but doesn't work properly due to a bug?