In Roy Koopa's room, the walls will close in a little bit after each of the first two times you stomp him. Stomp Roy near the middle of the room, then run at the left wall and keep pushing into it. Mario will go into the wall as it moves, and will stay in there as long as you keep holding Left on the directional pad. Roy won't be able to hurt Mario while he's in the wall, so this glitch might help if you're having trouble beating him.