After you have collected all the blueprints in the game, go to any Snide's HQ in any of the levels and bring up the Bonus Menu. I recommend highlighting Beaver Bother because if this code doesn't work, you can end that mini-game right away by jumping down the center hole. Now press and hold B and quickly press A. If you did it exactly right, you won't be taken to the mini-game...
Instead, you'll end up...here. It's a room with no walls, no music, four platforms, a yellow banana balloon, and a motionless clone of Donkey Kong.
He occasionally blinks his eyes, but that's it. You can run right through him. And he'll be here no matter what Kong you enter with, even if it's Donkey Kong.
If you come in here as Donkey, you can shoot this yellow balloon. But since you can't bring up the pause menu in here, it won't affect your totals.
All of these platforms have the same tile pattern on top except the tallest one, which has a pattern that matches the floor's.
There is no escape from this room. If you walk out into the black void, the game will act as though you went through a door and restart you in this same room. The only way out is to reset.
I wonder what purpose this room served, as it seems to have been intentionally programmed in. A remnant of the Beta version of the game? A programmer's area to test the game's mechanics? Something that wasn't finished (possible unfinished battle arena or bonus area, maybe)? I guess we'll never know...