Pikachu's ghost glitch, also known as awakening Pikachu after evolving it using the Q glitch is a glitch exclusive to Pokémon Yellow.
It involves putting a Pikachu/other Pokémon unstable hybrid to sleep using the Pewter Pokémon Center Jigglypuff, removing Pikachu in your party by evolving it with a Thunderstone, then awakening a non-existing Pikachu with the Poké Flute by moving at one step away from it or standing in the same spot as it first.
Effects
The "Z.....Z" speech bubble will appear from a Pikachu that does not exist, it will make a sleepy sound and the sleeping Pikachu box will appear.
Additionally, with the Pikachu's ghost glitch you can get the "It looks very content asleep." message from the Pokémon Center nurse when Pikachu isn't there.
Steps
For a Pikachu in Pokémon Yellow to be considered as a walking Pikachu, it must have its first species byte (donor byte) as Pikachu and have the same OT and Trainer ID as the player.
The second species byte gives Pikachu its voice on the summary screen and if it is hex:54 (Pikachu), a walking Pikachu will refuse a Thunderstone.
You can have hybridize a Pokémon with the same ID and OT as you with Pikachu using the Pokémon merge glitch, and the game will consider it as a walking Pikachu. This Pikachu (e.g. a Pidgey/Pikachu hybrid) will be able to take a Thunderstone because its first species byte (Pikachu, hex:54) makes it a walking Pikachu yet its second species is not Pikachu (so it can take a Thunderstone), allowing the glitch.
1) Obtain a Q (hex:FF). You can get one by evolving 44 Hy hex:C0 (obtainable with the Ditto trick with a Special stat of 192) at level 6.
2) Deposit Pokémon into an empty box in this order:
i. (Any Pokémon)
ii. Q
iii. Pikachu
iv. (Any Pokémon)
3) Withdraw Pokémon i. then withdraw Pokémon iv. Pokémon iv will be a Pikachu/(original Pokémon) hybrid.
4) Go to Pewter City Pokémon Center and talk to the Jigglypuff with the Pikachu/original Pokémon hybrid in the party. This will make it stay on the spot.
5) Use a Thunderstone on this hybrid to evolve it into Raichu. Do not leave the Pokémon Center.
6) Use the Poké Flute when you are at most one step away from where Pikachu was.