From Bulbapedia, the community-driven Pokémon encyclopedia.
The Mew glitch is a software bug in all three of the Generation I Game Boy games, discovered in 2003. This glitch is famous for allowing players to catch any non-glitched Pokémon in the game, most notably Mew. Method #3 of the Mew glitch allows one to catch both glitch Pokémon and regular Pokémon.
To get Mew at the earliest point possible in the game, the player must not have defeated the Swimmer in Cerulean Gym and the Jr. Trainer (the one who says "I saw your feat from the grass" when the battle with him begins) on Route 24 in the grass. The player must defeat the Trainers on Nugget Bridge as usual.
The player must have an Abra that can Teleport in their party, and therefore, if playing Pokémon Yellow, must have reached Route 5 to catch one or traded for one beforehand.
The player must first heal at the Cerulean City Pokémon Center, so they may Teleport there later. With an Abra in their party, the player then needs to stand north of the Jr. Trainer so that he is just below the screen. It is recommended that the player now saves the game. The player must move down until seen and immediately press the "START" button. The player then needs to Teleport with Abra. If done correctly, the Jr. Trainer should get the indicative exclamation mark above his head immediately before the player Teleports. At this point, the menu will not work until after the player enters a battle, though it is imperative to not battle.
The player then needs to head east from the Pokémon Center into the Cerulean Gym and battle the Swimmer. After defeating the Swimmer (the menu should be available upon victory), the player then must head to Route 24, and after several steps the menu will appear. Upon closing of the menu, a battle will begin with a wild Mew.
Method #1 (alternate method)
The player must follow the steps for Method #1 exactly until after teleporting from the Jr. Trainer.
The player then must head north to Route 25 and battle the Youngster. As in Method #1, it is imperative to avoid all other battles before then. The player must engage in battle by being seen (and must walk along the wall to be seen, not towards him, otherwise the game will freeze).
After defeating the Youngster, the player then needs to walk towards Cerulean City. After a certain amount of steps the Start menu will appear. Closing it should begin a battle against a wild Mew.
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For this method, the player needs to find any Trainer in the game that will engage in battle the moment the Trainer is on screen and the player is in the Trainer's line of sight. The player then needs to take one step down closer to the NPC, but make sure that the NPC does not see the player, and then immediately press the "START" button. The menu will appear and the player will be able to Fly or Teleport. The player needs to fly or teleport to Cerulean City. While flying, the exclamation point that usually appears above a Trainer's head when they challenge the player to battle will appear. The player then needs to go to Route 24 and, defeat the Youngster that owns a Slowpoke (and must walk along the wall to be seen, not towards him or the game will freeze). After defeating him, the player needs to fly or teleport to Lavender Town and head to Route 8. The menu will appear. Pressing B will make a wild Mew appear at Level 7.
- This method is also known as "Method #3".
For this method, the player first needs a Pokémon with a Special stat of 21. It doesn't matter if the stat was naturally leveled or boosted with stat experience, as long as it's not due to an in-battle Special-stat boosting skill. The player needs to find any Trainer in the game that will engage in battle the moment the Trainer is on screen and the player is in the Trainer's line of sight. The player then needs to take one step down closer to the Trainer, but make sure that the Trainer does not see the player, and then immediately press the "START" button. Once the player has pressed Start and the menu appears, the player needs to Fly or Teleport elsewhere. The location being traveled to must have a path via which the player can walk both to a place where there are wild Ditto (Route 15RB or Pokémon MansionY) and a Trainer to fight. This is because the Start button will no longer work until after a battle with a Trainer (to be mentioned) because the game has been tricked into thinking it's in battle mode. Once the battle with the Trainer the player picked (the first Trainer) was evaded successfully with Fly or Teleport, the player then needs to battle any Trainer in the new location; there needs to be one space between the player and the second Trainer when the battle is initiated so that the second Trainer has to walk up to the player. After defeating the Trainer, the player needs to encounter a wild Ditto, and battle it until it uses Transform on the player's Pokémon with the Special Stat of 21. After defeating the Ditto (with any of the player's Pokémon), the player must not battle, and go back to the location where the first Trainer the player first evaded is located. Once the location is approached, the menu will automatically appear. The player must press the B button. A battle will begin with a wild Mew.
This method can be used to catch any Pokémon in the game, even some glitch Pokémon. 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, ゥL ゥM 4 will appear instead of Mew. Some Special stats will cause glitch Trainers to appear.
Many different regular and glitch Pokémon, as well as glitch Trainers, can be encountered with the Ditto glitch. While Special stats cannot go below 5, special stats above 255 are reduced to modulo 256. Here is a table showing which special stat corresponds to which Pokémon. Glitch Trainers are in bold.
A Pokémon encountered using the Mew glitch is most commonly level 7. This is because as one Pokémon's Special Stat corresponds to the index number of the Pokémon, the Pokémon's level also corresponds to the attack stage of the last encountered Pokémon. The attack stage is the variable which is changed when a stat-modifying move like Growl is used. 7 is the unchanged value, but will range from 1 to 13 depending on how many attack boosts or drops the opponent received. For example, if the player used Growl on the Pokémon six times, he or she could encounter a level 1 Pokémon. However, Pokémon with levels 14 to 255 and 0 cannot be found this way, because a Pokémon cannot have more than six attack boosts or drops.
If the Ditto glitch is used and a glitch Trainer is encountered, the Trainer class will use a different team corresponding to the attack stage. Usually the game will attempt to load the 7th party in memory. Some classes only have one to three set parties. This causes glitchy parties to appear if an attack-modifying move is not used to lower the attack stage to at least 3.
Text box variations
If a player causes a text box to pop up (ex. talks to a NPC, reads a sign, etc) before they return to the route where they flew/teleported/dug away from the trainer, something other then the start menu will pop up.
Typically, instead of the start menu, dialogue from trainers on the route will pop up instead. Dialogue from right before you battle them, right after you win or dialogue from every time you talk to them afterwards, may come up followed by a wild Pokémon battle. If dialogue from right before you battle them comes up there is a good chance that you'll fight the trainer that it belongs to. Most of the time though, after defeating them, you will no longer be able to move.
Other text may come up as well. Sometimes signs or even item text will come up. If the the item text comes up the player will have said item and the item will disappear from the route if it was not picked up. However, every time item dialogue pops up the player will obtain the item even if it's not on the route already allowing the player to obtain multiples of the same item (though not an effective means as the player would have to do the entire trick multiple times and with a finite amount of trainers it could only be done so many times).
In some instances, however, instead of a text box a glitch will come up. The glitches may vary from something minor like glitch music, to something that effects the whole area like 0 ERROR or the Trick Zone.
The glitch in fact works as follows: Any Trainer can be used for the first section, 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). Teleport and Fly (or Escape Rope and Dig if in a dungeon) can be used to trigger the glitch. For the Youngster, any Trainer will do as long as they can be fought without walking up to them. To finish off the glitch, the player must return to the route of the Trainer 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 stat the player needs and finding a Ditto that transforms will produce an identical effect. The required Special stat for Mew is 21.