The overworld sprite trick is a glitch technique in Pokémon Red and Blue (English), requiring the 255 Pokémon glitch. It enables the player to very briefly have a menu sprite resembling a glitch Pokémon or another general illegible glitched sprite.
Steps
1) Clear your save data and start a new file.
2) Save and reset shortly after the Yes/No box disappears.
3) Check your save file to see if you now have 255 party Pokémon.
4) If successful, swap the second Pokémon with the tenth to get 255 items, and walk into the tall grass.
5) Battle with Blue, and use the Awakening at slot 41 on Pokémon 1.
6) After the turn ends, use another Awakening at slot 41 on Pokémon 13.
7) Switch to Pokémon 13, switch it out, and battle with it to win.
Two Pokémon may grow to level 4, the Pokémon that's out may grow to level 1, and another Pokémon may grow to level 58.
Red will turn into a 'glitch character' until you press A, and you get to see a loop of error codes including "14 ERROR.", "19 ERROR.", "20 ERROR.", and a stray "0".