The Select glitch, among its many uses, can be used to walk through walls.
- Having at least 3 Pokémon.
- Having at least 13 items.
- Highlight the 13th item with Select.
- Exit with B,B, enter battle and select the third Pokémon from the ポケモン menu to swap it with a glitch Pokémon.
- Run from battle. Open the items pack menu. You should have over 20 items.
- Highlight the 32nd item with Select and exit the menu with B,B enter battle and select the third Pokémon from the ポケモン menu to swap it with a glitch Pokémon again.
- Exit the battle and the bicycle theme should play.
- Jump off a ledge and you'll stay in mid-air and will be able to walk through walls.
YouTube tutorial by ChickasaurusGL