Son of a Glitch!

[E-Man]

Long before I even knew how to browse the Internet, I discovered a glitch involving the castle tower in Super Mario 64. With the help of the Wing Cap and the cannon you'll get access to after collecting 120 Power Stars, fly to the central tower and land on the bit of roof on the lower tier. Not only is it possible to get to the top of the tower this way (just run up the left wall), but also you'll be able to clip right through it. If you force Mario to press against the walls (in particular, the diagonal ones), you'll fall clean through the tower and end up in a room inside the castle behind the main doors. On top of that, if you enter the doors from this room, you'll end up inside that actual interior of the castle,but you'll end up in another room similar to the one you are in before. Simply jumping at the wall that borders the door in this new room will return you to the actual playing area.

I wouldn't be surprised if you guys know about this glitch already.