Pokemon Red/Blue Glitch: Level 100 Mew

Step by step:
Heal your team at the Cerulean City Pokemon Center
Head up Nugget Bridge
Go left until you're right in front of the first patch of grass
Take one step south and immediately press start
Use Abra (or any other Pokemon that knows Teleport) to Teleport back to the Pokemon Center
Battle the first trainer in the Cerulean Gym
Defeat his Horsea
Use Growl on his Shellder at least 6 times (Each Growl reduces Mew's level)
Go back up Nugget Bridge until the menu opens itself
Close the menu and Mew will appear at LV1
Use a sleeping attack to put Mew to sleep
Throw Pokeballs at Mew only if he's asleep (He will dodge almost every ball if he's awake. This will take a few tries.)
Make Mew your team leader
Find a wild Pokemon to fight
Switch Mew out for a Pokemon that can defeat the wild Pokemon
As long as Mew gains less than 54 experience points, he will jump straight to level 100