A lot of old video games (especially RPGs and really early arcades) would have these things that came to be known as "programmer's rooms". They were basically just easter eggs that were implemented as special screens integrated into the game.

The best programmer's room ever was in Final Fantasy IV. It was a real room, a part of one of the castles. In the dwarf castle you first come to in the underworld, look closely between the inn and the shop. There's a weird little hole in the wall behind one of the pillars. Go into it, and then look closely for some cracked floorpieces in the room you find. The wall next to the cracked floorpieces can be walked through. Walk through it. I'm not going to tell you where you wind up, and i'd like to request nobody post writeups on this node spoiling it, but it kicks ass.

(Ai! Ursus is, of course, right. I forgot. Sorry.)