Inside the stairs at Hooktail's castle

From Glitches

From the game Paper Mario 2 for the Nintendo Gamecube

You must have Yoshi to do this. Go back to Hooktail's Castle again, and near the beginning there will be stairs and a pink or red button above it. Pushing the button switches the stair's direction. Take Yoshi to the ledge, and hover to the button from either side. Jump off Yoshi, and you will automatically hit the button and jump off. When the stairs flip, it will flip on top of you and you will be in the stairs.

Submitted by: Deathknight