Please note before going through this that the method used to obtain Celebi has a chance of being harmful to the cartridge, as one of the requirements for this is a Bad Clone. I am not responsible in any way for damage done to someone’s game for attempting this. Anyways, if done correctly the result of these steps will result in the player getting their very own Celebi, using some hidden hex values. Anyways before you begin, you will need three things: An egg that will hatch a Pokemon knowing Beat Up as its third move (the third move being important), a Bad Clone, and any five unneeded Pokemon. The easiest way to get the egg is to breed two Pokemon together with the same move set, such as Sneasel. The reason Beat Up is needed is because its assigned hex value is 251, the same as Celebi. This essentially means that you can produce any Pokemon in the game with this method (on the exception of Ledyba, because the move that shares its number is Struggle, which can’t be learned). As for the Bad Clone, there are different methods to getting one. The Box Clone technique can be used to obtain one, though it may take several tries. The ‘successful’ clones can be used as the third requirement.
Next comes the steps. Drop the egg with Beat Up into a box, with the five unneeded Pokemon and the bad clone in the party. Next, head for the daycare in Goldenrod City, and drop the Bad Clone into the Day Care. Following this, ask for it back. The Bad Clone will be turned into a ?????, a different type of glitch Pokemon. As a safety precaution, finish the rest of the steps without trying to view this Pokemon’s info or Pokedex. Move ????? to the top of the party, then log onto the PC. From there you will need to grab one more unneeded Pokemon using the ‘Move Pokemon Without Mail’ option, and place it at the top of your party. At this point you will have seven Pokemon in your party, the newly withdrawn Pokemon being the first, followed by the ?????, then the other five.
It’s at this point that things may start to become a bit more complicated. Return to the daycare with the seven Pokemon, deposit the ????? back into the daycare, then go back to the PC. From here, you will need to deposit the first and second Pokemon , then use the ‘Move Pokemon Without Mail’ option again, to grab the egg and place it at the top of the party. Following this, deposit the rest of the Pokemon except for the egg into the PC, then withdraw any Pokemon that were not designated for use in this glitch. With a full party of an egg plus five, proceed to move around (preferably in a place where no Pokemon battles will be encountered), until the egg hatches into a Celebi. You’re not done here however, as the Celebi will be hatched at level 0, meaning that it cannot be levelled up normally. In order to fix this, the Celebi will need to be dropped into the Daycare until it becomes at least level 2.
After Celebi has hatched, the ????? should disappear from the daycare, if the steps are followed properly. However there is the odd occasion where it may still remain. The problem with ????? Is that it cannot be released through the use of the PC, so the only way to remove it altogether is to trade it to an older generation (Red, Blue, or Yellow), and dispose of it from there.
Once you become more comfortable at using this glitch, you can take it a step further, and with research create items as well. While the third move of the egg determines the resulting Pokemon, the fourth move will determine the item held by the hatched Pokemon. If Reflect is the fourth move for example, the resulting held item will actually be the GS Ball, which can be used to trigger the event in the Ilex Forest to summon Celebi.