Most of the game is wandering around this mansion sucking up ghosts with a vacuum cleaner (yes, you heard me right.) The end boss fight happens on the roof - there's Luigi, being his usual scared self, and suddenly BOWSER drops down behind him, all twenty feet of armor, spikes, and flame breath. And here's Luigi, armed with his trusty vacuum cleaner. But anyway - you manage to suck up a bomb and hit him with it, and his head pops out. Turns out it's not the real Bowser after all, it's just a robot - after the pilot of the robot (a ghost, of course) flies around for ten seconds or so, he goes back into the robot, it gropes around for its head, slams its head back on its body, and comes after you. Rinse repeat.
Well, I've done this a few times, and he stomps back on the screen (I've run away a bit to get some breathing room) and I just start laughing.
He accidentally put his head on backwards.
It's little touches like that that make this game so wonderful - it's too short, it's too easy, it's full price, and it's some of the most fun I've had in a *long* time.