Lift Key above ball glitch

Bulbapedia also has an article about Lift Key above ball glitch.

The glitch in action

The Above Lift Key ball glitch is a glitch in Pokémon Red and Blue that allows Red to be standing on top of the item ball that contains the Lift Key in the Rocket Hideout.

The player simply needs to defeat the Rocket Grunt, then talk to him again from his left. The item ball will appear under Red, who can still move.

In Pokémon Yellow, the Lift Key drops as soon as the Grunt is defeated, so the glitch isn't possible anymore without cheats.