Invisible Rocket glitch
The invisible Rocket glitch is a glitch derivative of walk through walls or wrong warping in Pokémon Red, Blue, and Yellow.
On Nugget Bridge, there is a Rocket disguised as a normal citizen who challenges you after you beat all five other Trainers on Nugget Bridge and you normally cannot avoid him.
If you use a walk through walls glitch to bypass him and avoid his battle, help Bill and obtain the S.S. Ticket, he will become invisible. However, walking where he would spot you will still activate his text and the battle.
One way to perform this glitch without cheats is to perform the 255 Pokémon glitch (resetting with no data after pressing save shortly after the Yes/No box disappears), warp to Cerulean City (by swapping Pokémon 1 with Pokémon 10, toss the quantity of item 36 until it is 03), and then enable walk through walls.
- ChickasaurusGL (documentation, article text)