This glitch lets you flood all of the Tall Tall Mountain level from Super Mario 64. It works in any version of the game. There are at least two ways to do this.
Go th TTM as Yoshi. Go up to that cloud that blows you away (I can't remember the name) and eat it. Go down to the bottom of the mountain, and teleport on the small mushroom. When you rematerialize, the whole level will be underwater! If you swim too high, you will appear to surface, but will still be underwater. You can still dive at this point. Jump from there, and all the water will disappear, and you might fall to your doom. The only real application of this is to swim to the lonely mushroom. Be sure not to get caught in the wind, as it will make you fall to certain doom.
As anyone, get on the owl. As you are at the peak, and over the patch of mushrooms, fall. Make sure to fall on the land NEXT TO the mushrooms. You will lose half your health. Teleport, and you get the same results as above.
Why it Works Edit
Possibly an easter egg planted by Nintendo.
METHOD ONE: Anonymous
METHOD TWO: Found on a GameFAQs FAQ.