Battle disabling Select glitch | Badge describer glitch | Dokokashira door glitch | Fossil conversion glitch | Item creation glitch | Lift glitch | Partial switch glitch (Red/Green v1.0 only) | Remaining PP into species Select glitch | Second type glitch | Skip to Level 100 glitch | Trainer mutation glitch | Walk through walls | Warp by Nidoran♂ glitch
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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