Factual error: In the fourth floor of the Celadon Department Store, there's a guy who says that he's getting a Poke Doll for the Copycat of Cerulean City. Too bad that the Copycat actually is from Saffron City.
Other mistake: At the beginning of the game, Oak gives you a Pikachu. By this time, Pikachu is in a Pokeball. When you fight against Gary, however, the Pikachu doesn't come out of a Pokeball, like in the rest of the game. However, in this battle, he should come out of his Pokeball.