YOu probably know of this glitch, I found it out long long long long long long long ago.
You have to be as far as Cinnibar Island atleast.
Red and Blue only:
Talk to that guy in Veridian City that teaches you how to catch a pokemon. Watch him catch the pokemon. After that, fly or walk to Cinnibar. (Flying's faster Tongue) Go to the edge of Cinnibar right on the right, just so that your touching the edge of the island, but your not on land. Go up and down and up and down on the edge of Cinnibar. You will eventually find a Pokemon Named Missigno, he will just be a blurr pokemon. His name will be Missingno or ∆M˚ or something like that. Kill him. DONT CATCH THIS GUY, HE WILL INSTALL A VIRUS ONTO YOUR GAME! THIS IS A DANGEROUS GLITCH IF YOU DECIDE TO CATCH HIM!!! After killing him, go up and down at the edge again. YOu will eventualy start to find pokemon that are levels higher than level 100. You can catch these ones. When you fight with them, they will level down to level 100. On Blue, Ive only been able to find Magnemites that were level 120 in this glitch which evolve after leveling up to 100. In red, you can find secend evolve forms of many pokemon that even require stones, You can often find Starmies on Red and Arcanines and Pidgeotos. This WONT work on Yellow, Nintendo fixed this glitch in yellow.
From what I know, this glitch was made from an error in the hex codes. The hex goes to 255, but there are only 151 Pokemon, so that causes a problem. When you go to the old man and he shows you him catching a Pokemon, it changes the name to "Old Man" so it screws up some codes. The values of the letters you use in your name will determine the Pokemon that will appear in this glitch. The name "Missingno" meens "Missing Number" (Missing No.) because its appearing as a character above 151, which doesnt exist.
Or maybe it was just spilled Coffee...