Action DooM! Easter Egg - Doom Kart - Out of Order

on the level in the underground UAC base (use the idclev3 cheat to save time) play as normal until you reach the point where a failed BFG firing test blows several holes in the walls. Go through the right hole into the cantine.

at the other end of the room are a group of arcade games, and among them is an arcade game based on the expected doom wad 'Doom Kart'. Notice that there is a sign on the screen reading 'Out of Order'.

This is a rather cryptic nod to the fact that DooM Kart was discontinued before it could be released, disappointing many.