Mew glitch
The Mew Glitch is a software bug in the Generation I Game Boy Games. This glitch is famous because it allows you to get catch any Pokémon in the game, most notably Mew, without a GameShark or other hacking device -- and method #3 allows you to catch ANY Pokémon in the game (even rare, previously unknown glitchy Pokémon such as LM4 and X - x.)Below are the methods on acquiring Mew in the Generation I Game Boy Games.
Method #1
To acquire Mew at the earliest point possible in the game, one must not have defeated the Youngster with the Lv. 17 Slowpoke on Route 25 or the Junior Trainer (the one who says "I saw your feat from the grass" when you begin a battle with him) on Route 24 in the grass. It is prudent to save before doing this. Defeat Nugget Bridge as per the usual, battling your way to the Youngster but do not engage a battle with him.
Return to Route 24 and catch or trade over an Abra. Place the Abra in your party if it isn't, then return to Route 24 if you were not there already. Stand in front the Junior Trainer, but out of range of his sight. Move down until he sees you, but when he does, immediately press START. Then use Teleport with your new Abra. If done correctly, the trainer should get the indicative exclamation mark as you Teleport.
Notice your menu will not work. Head North to the Youngster and battle him. Do not battle anything or anyone else or talk to anyone before you do. This includes the Youngster. You must engage in battle by him seeing you, not by walking towards him. This will cause a lock-up.
After defeating the Youngster, Teleport to Cerulean City. Prepare yourself with Poké Balls and go North to the Nugget Bridge. Once you reach the bridge, your start menu should appear. Press the B button and you should begin a battle against a Lv. 7 Mew, which is not difficult to catch.
Method #2
Go to the Gambler in front of the Underground Tunnel entrance east of Saffron City. Place yourself north of him so that he is just off screen. Take one step south closer to him, but make sure that he does not see you then immediately press START. If he spotted you, try again so that he doesn't. If it was successful, the menu will appear and you will be able to Fly, Dig, or Teleport. Use Fly to reach Cerulean City. While you are flying, the exclamation point that usually appears above a Trainer's head when they wish to battle you will appear. Go to Route 24 and defeat the trainer that owns a Slowpoke. Have him walks towards you to battle or the game will freeze. Defeat his Slowpoke, then Fly to Lavender Town and head west to Route 8. Mew will then appear.
Method #3
Go to the Gambler east of Saffron City or either the Biker west of Route 14 or the one just below him (there are two of them together). Have the trainer go off screen then walk up to him and immediately press START before the trainer spots you. Fly to Fuchsia City and go east towards Route 15. Make sure a Pokémon with a Special Stat of 21 is with you. Place that Pokémon first in your party then battle any trainer in the game. Defeat them and then search for a wild Ditto. Keep yourself in battle with it long enough so that it uses Transform on your Pokémon with the Special Stat of 21. Then defeat the Ditto. After defeating it, avoid running into any other wild Pokémon and/or battling any other trainers. Go back to the Route where the trainer you first evaded is located (for the Gambler, east of Saffron City, for the Bikers, Route 14). Press the B Button and Mew will appear. This method can be used to catch any Pokémon in the game, even Missingno. in the Yellow version. For example, in the Red/Blue versions, if a Pokémon with a special stat of 198 is used, a glitch called "LM4" will appear instead of Mew. (See [1])
(Important note: in the Red/Blue versions, Ditto will be found at Route 15. In the Yellow version, Ditto will be found in the Pokémon Mansion's basement located in Cinnabar Island. Because of this, players in Yellow version should not go to Route 15, but instead head to the Pokémon Mansion's basement. It is highly recommended to prepare yourself with an Escape Rope or Repel to use soon after finding a wild Ditto so that the player will not fail the glitch by running into the wrong Pokémon.)
Overview
The glitch in fact works as follows: Any trainer can be used for the first section (Junior Trainer), as long as the trainer can be engaged with by them getting an exclamation point above their heads. Any movement move (Teleport, Dig, Fly) can be used to trigger the glitch. For the Youngster, any Pokémon will do as long as it can be fought without walking up to a trainer. To finish off the glitch, you must return to the route of the trainer you "evaded." The Pokémon that you will find wild is dependent on the Special stat of the Pokémon you last faced (wild or from the trainer), and so using a Pokémon with the Special you need and finding a Ditto that transforms will produce an identical effect. For the record, the required Special stat for Mew is 21 (This is based on hexadecimal numbers, so you can get the appropriate number for a different Pokémon from a hacking device code list, then translate it to decimal).
