Adding to what Shiny Hunter said, here's the glitch-
Requirements- Three Badges(of course)
HM02(Fly)
You haven't battled a Youngster looking up(4th trainer) on Route 25 that leads to Bill's House, and a Gambler facing the entrance to the Underground Path on Route 8 from Lavender
About 7 Great Balls
From Celadon City, go into the underground path connecting Celadon and Lavender, and go outside. Before you move, SAVE RIGHT AWAY, then take one step, and as soon as you take it, hit START and Fly to Cerulean City. The ! symbol will pop up but you'll Fly away :D.After that, the START button won't work for a while. Now go up to Route 25 and battle the Youngster mentioned above. NOTE: Do not walk up to him, or the game will crash. Go all the way up so he has to walk up to you. After you beat him, the START button will work again. Fly to Lavender Town, and go straight to Route 8. As the song changes, the START MENU will pop up by itself, so press B to take it off. And voila! A lv.7 Mew pops up, and it only knows Pound. Enjoy!
PS- When you go to Lavender Town after battling that Youngster, do not go to the Mart to buy the balls, or you will either battle a Super Nerd for life:(, or your game will crash, so buy the balls in Celadon before trying the glitch.