Mew glitch
The Mew Glitch is a powerful glitch in the original (Generation I) Games. This glitch is famous because it allows you to get catch any Pokémon in the game, most notably Mew, without a GameShark or other hacking device.
To acquire Mew at the earliest point possible in the game, one must NOT have defeated the Youngster on Route 25 or the Junior Trainer on Route 24 in the grass. I suggest saving before doing this Defeat Nugget Bridge as per the usual, then fight your way to the Youngster but DO NOT FIGHT HIM!!! Return to Route 24 and catch an Abra. Place the Abra in your party, then return to Route 24 if you weren't there already. Stand above 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 point as you teleport. After finishing that, head north to the Youngster and have him spot you and then defeat him. Do not fight anything else, or talk to anyone before you do (This includes the Youngster! You MUST Engage in battle by him spotting you, and don't even walk up to him either. For some reason that causes a lock-up). After defeating the Youngster, teleport back to Cerulean again. Head north onto Nugget Bridge with some Balls, and when you get to the bridge, your start menu should pop up. Press B and you should engage in a fight against a levev seven Mew, and it is quite easy to catch really.
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. For the Youngster, any Pokémon will do as long as it can be fought without walking up to a trainer. The Pokémon that you will find wild is dependent on the Special Stat of the Pokemon 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.
