The Mew glitch can be used for (most commonly) obtaining Mew, usually only obtainable at Nintendo special events, as well as almost any Pokémon, including glitch Pokémon such as Missingno..
The Mew Glitch is done by interrupting a battle to trick the game into summoning the Pokémon of your choice. You can get any Pokémon from this glitch, but it is called the Mew Glitch because it's the only way you can get Mew, who was once only available from special Nintendo promotions.
- To perform the glitch, first make sure you have not battled/defeated these two trainers:
- Get a Pokémon with FLY, and put that Pokémon in your party.
- The Mew is a level 7 Pokémon; plan accordingly. It is also advisable to bring Ultra/Master balls, as opposed to normal Pokéballs.
- Get to a trainer nearby. Save before the trainer is on screen. Don't make him/her battle you yet. Now, step one step towards the trainer, and quickly press START. If you performed it correctly, the start menu will pop up, but the trainer won't battle you. Next, FLY to anywhere there is a trainer.
- If you've done this right, after you use FLY, the trainer you FLEW to should want to battle. You cannot use START, A, or B at this time.
- Do NOT save anywhere after this. You cannot save while the buttons are disabled, and besides that, saving negates the glitch, and the important trainer will be defeated, not allowing you to get the Mew. If you are using a Pokémon with the Special, and you haven't battled the Ditto yet, you CAN save. If not, keep your saved file from before you battled your chose trainer.
- Walk (you can't use START) to the second trainer, and when you battle him, make sure he WALKS TO YOU. Do not go directly in front of him. Beat him, and (now that your buttons work) FLY to near where your chosen trainer was.
- Now, go to the Route where you battled the first trainer. Do NOT save here. Press B. Mew will appear.
- First, make sure you have not beaten these trainers:
- Then, go in front of the Gambler. You must approach him from in front of the house, then FLY quickly before he sees you.
- Then go and battle the boy. Make sure he walks up to you, if you go too close to him, the game will freeze.
- Then FLY to Lavender Town.
- Press B when the menu pops up.
- Mew should appear.
The user can additionally catch any of the other 150 Pokémon with this glitch.
Can I get Mew a lot?
First you have to do Lavender one first except do not beat the Youngster with Slowpoke, let him beat you instead. When you see Mew, he won't be level 7, but a random level like 2 or 5. After you catch Mew, do the Lavender trick. Do get beaten by the Youngster all the time and when you see Mew, weaken him and capture him.
http://www.gamefaqs.com/portable/gameboy/file/907714/23116 - TheScythe's guide
http://glitchcity.info/docs/ditto/mew - A Pokemon glitch website