From Glitch City Laboratories
The Glitched Pokédex is a glitch that allows you to see Pokémon past #151.
To perform this glitch, start a New Game with the Gameshark code "01091ED3" on, go up to your PC before going downstairs, get out a Potion, then open your item inventory and use the Pokedex in it. You won't see any Pokemon until you scroll down past 151. There, you can see a few real Pokemon, and some glitch Pokemon too. For the glitch Pokemon, their viewing sprite and the cry that is heard when "CRY" is selected are different. They have no data, though. Nothing has been caught.
There aren't many Pokemon, yet it is a bit interesting.
Credit goes to Dement for finding this glitch. (Original forums thread).