Okay if you got here, chances are you want to know how to get the secret and can't be bothered to detect it yourself.
Here it goes. Click Cancel 8 times. You'll hear wolfenstein 3d solder saying "Schutzstaffel". After that you'll get access to some wolf 3d textures, and also rat will turn into miniature ratte tank.
Then if you'll set all textures to represent wolf 3d first level ones (new grey bricks for wall, dark solid grey for ceiling an bright solid grey for floor) you'll get SS saying "Mein leben", "get psyched" loading label, it will start loading and then.....crash game to BSoD, with abilty to ESC back to menu.
Truth to be told i wanted to make a fully working wolf3d level. But then i thought about time i'll spend writing all the systems - hud, weapons, damage, etc, and time constraints and decided "Fuck it!, it'll just bsod".