Okay, for anyone wondering how to get Mew, here's how, this works on Red, Blue and Yellow. First, do not beat the kid with the single Slowpoke on the way to Bill's, also, don't beat the gambler staring at the underground passage between Lavender and Saffron.
Now, once you have fly, go to the underground passage, and save, in case you do this wrong. Make sure you're right at the door, then take one step down (should be in the Gambler's field of vision, and make sure he sees you, if he does, then load, and you'll know you're in the right place, this is just to get it so you're one step ahead of his field of vision.
Now that you're one step ahead, walk one step down, and pause. If you've done this right, he will not have seen you. Next, Fly to Cerulean. You should see the Gambler get the exclamation mark, then you fly away. Walk up to the kid with the Slowpoke, and walk into his field of vision, ENSURING THERE IS ONE OR MORE STEPS BETWEEN YOU! If you are right up to him, your game will freeze. Now, beat him, and Fly to Saffron. Start walking toward the underground passage, and you'll be stopped by your start menu popping up. Hit B to close it, and a level 7 Mew will appear.