In Donkey Kong 64, it is possible to enter a secret test room by obtaining all 40 Blueprints and then pressing A+B simultaneously while accessing Snide’s minigame menu. The room contains nothing but four pillars, a yellow Banana Balloon, and a model of Donkey Kong that is motionless save for his blinking eyes. The Donkey Kong model is not solid and can not be interacted with. Entering the darkness outside the room activates the room exit transition, but then simply loads up the room again. The only way to escape the room is to turn off the console. (Source)