Mew glitch
The Mew Glitch is a software bug in the Generation I Game Boy Games. This glitch is famous for allowing players to get catch any Pokémon in the game, most notably Mew, without the use of GameShark or other hacking device. Method #3 allows you to catch any Pokémon in the game (even rare and previously unknown glitch Pokémon such as LM4 and X - x.)
Methods
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 towards Route 25 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.
(Note: You must battle the Slowpoke trainer on Route 24, but you do not have to battle the Gambler in front of the underground tunnel. Any trainer will do. One option is to battle one of the two bikers who are next to each other on Route 14, then battle the Slowpoke trainer, then return to Route 14. Because Route 14 is isolated, players should use repel to prevent from running into Pokémon in the grass that would mess up the trick.)
Method #3
Go to the Gambler east of Saffron City or either the Biker in western 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. When the menu appears, 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. Make sure the trainer walks up to you, and that you don't approach the trainer directly. Defeat the trainer and then find 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. While a Pokémon with a specal stat of 21 will give you Mew, using a Pokémon with a different special stat will give you a different Pokémon. 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])
It is important to note that in the Yellow version, wild Ditto cannot be found at Route 15. Instead, Yellow version players should follow all of the steps above, but instead go to the Pokémon Mansion's basement located in Cinnabar Island. This is because Ditto can be found there in the Yellow version. Also, Yellow version players must bring an Escape Rope or Repel with them to use immediately after defeating 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).
