This. It actually relates to the old man who teaches you how to catch a Pokemon in Viridian city. What happens, when you talk to the old man, and he battles the caterpie, it swithces the trainer's name with OLD MAN. Due to the limited amount of memory available on the game boy, they decided to store the trainer's name in the variable that decides what kind of wild Pokemon would appear along the coast. Usually this data would be overwritten, but if you go directly to Cinnabar Island, and surf along the coast, you would encounter Pokemon based on your name: Example, most people have used the name ASH, whose hexadecimal values would produce MissingNo. There are ways that you can set up your name so that only Mewtwo would appear there (I can't remember the exact name.)
