From Glitch City Laboratories
The Mew Trick is the first variation of Ditto Trick discovered, and it allows the player obtain a Mew. In fact, because of this, Mew is often associated with this trick.
1. The player must not have beaten any of these trainers who are circled in the picture.
2. Go in front of the Gambler shown in the picture. The player must approach him from in front of the house, then Fly quickly before the player is seen by him.
3. Go and battle the Youngster in the maze to Bill's house. Make sure he walks up to the player, as if the player goes too close to him, the game will freeze.
4. Fly to Lavender Town.
5. Walk back to the route to the left.
6. Press B when the menu pops up.
7. Mew will appear, at level 7.