Question

what is the mew glitch

Comments

https://m.bulbapedia.bulbagarden.net/wiki/Mew_glitch

Answer 1

The Mew glitch is a glitch in the mainstream Generation One games that allow you to get not only Mew but all the other 150 Pokémon in the game. Follow the below steps to do the glitch:

1. You must have not battled the Gambler on Route Eight and the Youngster on Route Twenty-five.
2. Head to Route Eight and stand directly beneath the door to the Underground Path, at which point the Gambler (circled below) is exactly one tile offscreen. It is recommended to save now in case you make an error in executing the glitch.
   
3. To initiate the glitch, take one step down while simultaneously holding down the Start key. You should see the Gambler and you will enter his line of sight, but the Start menu should appear before the Gambler “sees” the player.
4. From the start menu, fly away to Cerulean City. If you have done the previous steps correctly then the Gambler will have the indicative exclamation mark appear above his head, but then the Fly animation will begin before he can walk up and challenge you.

5. Upon arrival at Cerulean, the A, B and Start keys will not function properly as the game believes you are about to start a battle. From here, head up to Route Twenty-five and battle the Youngster (circled below). You know that you have got the right person if they have a Slowpoke on their team. Importantly, the Youngster must walk up to you (at least one tile between you and the Youngster) else the game will softlock.
   

6. After battling the Youngster, the aforementioned buttons will work again. From here, fly to Lavender Town and head west towards Route Eight. Upon entering the route, the Start menu will appear by itself; after closing the menu you will initiate a battle with a Mew at Level Seven.

Source

Comments

This is pedantic, but the Mew glitch is when you use the trainer escape glitch to encounter Mew. If you encounter something else with it then it's not the Mew glitch.

For those interested in why this works: it abuses a minor oversight with how the overworld map's event handling is updated, tricking the game into triggering an encounter when you return to the route. When the game tries to read info for the Pokemon you encounter, it reads arbitrary data for the Pokemon most recently battled, because the glitch causes the game to skip the event handler that normally writes over that data.
The species of your encounter will align with the Special stat of the last Pokemon battled, so we just have to battle a trainer with the properties we want, then step back into Route 8 (the map we choose for the glitch). Mew's index number is 21 and the Youngster's Slowpoke has a Special stat of 21 -- and so that youngster became famous.

Clearly, you can pick a different trainer that will load different numbers into memory, causing you to encounter a different species.