The Test Room is an Easter egg found in Donkey Kong 64. To access this Easter egg, you must have found all 40 of Snide's Blueprints within the game. Visit Snide's H.Q. and press C-Up to bring up the bonus mini-games menu and then simply press A and B together. It will then take you to a testing room that was meant to test out the game mechanics. Once you enter the room, however, you are unable to escape and the only way to get out is to reset the game.
The room contains four climbable pillars, a yellow Banana balloon and if you walk out into the black boundaries, you are respawned back into the room. The most eerie thing of all is that there is a motionless DK clone in the room, which does nothing but blink every so often.