Mew glitch
The Mew Glitch is a software bug in the Generation I Game Boy Games, discovered in 2003.[1] This glitch is famous for allowing players to catch any Pokémon in the game, most notably Mew, without the use of GameShark or other hacking device. Method #3 allows one 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, the player must not have defeated the Youngster with the Lv. 17 Slowpoke on Route 25 or the Swimmer in Cerulean Gym, and the player must not have defeated the Junior Trainer (the one who says "I saw your feat from the grass" when the battle with him begins) on Route 24 in the grass. It is prudent to save before doing this. The player must defeat Nugget Bridge as usual without engaging a battle with the Junior Trainer.
Before attempting this glitch, it is ideal to have several Poké Balls stocked up. Note that the player must have an Abra that can teleport, and therefore cannot be playing Yellow version unless he or she has reached Route 5 or traded for one prior to attempting the glitch.
The player first heals at the Cerulean City Pokémon Center, so they may Teleport there later.
The player then needs to return to Route 24 and catch (or trade over) an Abra, or withdraw the Abra if it is in the PC, then return to Route 24 if not already there. The player then needs to stand above the Junior Trainer so that he is just offscreen. Then the player needs to move down until seen, and immediately press START. The player then needs to use Teleport with Abra. If done correctly, the trainer should get the indicative exclamation mark while the player teleports.
At this point, the Start menu will not work. The player then needs to head north towards Route 25 and battle the Youngster. It is imperative to not battle anything or anyone else or talk to anyone before then, including the Youngster. The player must engage in battle by him seeing them (and must walk against the wall to be seen, not towards him or the game will freeze).
After defeating the Youngster, the player then needs to walk towards Cerulean City. After a small amount of steps, the Start menu appears. Pressing the B button should begin a battle against a Lv. 7 Mew.
If used on other trainers other than the Youngster, it will often be a different Pokémon (depending on the trainer's Pokémon's Special stat); for instance, doing so with the Lass below the Youngster will result in finding a Lapras, which is normally only avaliable at Silph Co.
Method #2
For this method, the player needs to go to the Gambler in front of the Underground Tunnel entrance east of Saffron City, and stand above him so that he is just off screen. The player then needs to take one step down closer to him, but make sure that he does not see the player, and then immediately press START. If the player is spotted, the glitch would fail. If it was successful, the menu will appear and the player will be able to Fly, Dig, or Teleport. The player needs to use Fly to reach Cerulean City. While flying, the exclamation point that usually appears above a Trainer's head when they wish to battle the player will appear. The player then needs to go to Route 24 and, just like in method #1, defeat the Youngster that owns a Slowpoke (with him walking towards the player to start the battle, or the game will freeze). After defeating him, the player needs to Fly to Lavender Town and head west to Route 8. Mew will then appear.
(Note: The player must battle the Slowpoke trainer on Route 24, but the player does not have to battle the Gambler in front of the Underground Path. 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 running into wild Pokémon in the grass and messing up the trick.)
Method #3
This method is also known as the "extended Mew glitch".
For this method, the player needs to go to the Gambler east of Saffron City or the Biker in western Route 14 or the one just below him (there are two of them together). then the player needs to get the trainer off screen then walk up to him and immediately press START before he spots the player. When the menu appears, the trainer needs to Fly to Fuchsia City and go east towards Route 15. the player needs to have a Pokémon with a Special Stat of 21 first in his party, and then battle ANY trainer in the game, as long as the trainer walks up to the player, and the player doesn't approach the trainer directly. After defeating the trainer, the player needs to find a wild Ditto, and battle it until it uses Transform on the player's Pokémon with the Special Stat of 21, and then he/she needs to defeat the Ditto. After defeating it, the player needs to avoid running into any other wild Pokémon and/or battling any other trainers, and go back to the Route where the trainer the player first evaded is located (for the Gambler, east of Saffron City, for the Bikers, Route 14). After pressing the B Button Mew will appear.
This method can be used to catch any Pokémon in the game, even Missingno. in Yellow Version. While a Pokémon with a special stat of 21 will give one Mew, using a Pokémon with a different special stat will give the player 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, Repel, or a Pokémon with teleport 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 notices the player as soon as he/she enters the game screen, and that the player is walking towards them from either the top, left or right (walking from below will not work). Any movement move (Teleport, Dig, Fly) can be used to trigger the glitch, or Escape Rope if in a dungeon. 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, the player must return to the route of the trainer he/she "evaded." The Pokémon that the player will find wild is dependent on the Special stat of the Pokémon the player last faced (wild or from the trainer), and so using a Pokémon with the Special the player needs 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 one can get the appropriate number for a different Pokémon from a hacking device code list, then translate it to decimal).
