Mew glitch. Defeat nugget bridge, DO NOT ENCOUNTER THE TRAINER LOOKING NORTH ON THE GRASS WHERE ABRA SPAWNS, catch an Abra, be sure to visit the pokemon center in cerulean city before this next part. Line up directly north of the trainer by the Abra spawn, just one block out of his line of sight. Press down and start at the same time. If you did it right, the start menu opens and the trainer is visible in the bottom of the screen. Use teleport on Abra, returning to cerulean city. Go into the gym, defeat the first swimmer, then return to nugget bridge. Upon entering the route, a battle should start and that'll be your mew. Catch it.