Other mistake: Being in any route or cave, and then flying or transporting to Cinnabar Island will make the Pokemon of that route and cave appear in Cinnabar's eastern coast, even though they're not water Pokemon and of course don't belong there. This can apply to Safari Zone's Pokemon also, that is, you can go to the Safari Zone, waste your steps there, fly to Cinnabar Island, surf in the coast, and you'll find the Zone's Pokemon there, which can be caught normally, and not with the methods of the Safari Zone (rock, bait).
Other mistake: Go to the Day-Care Man. When he asks you if you want him to care a Pokemon for you, answer NO. He'll say "come again." However, since it is the start of a sentence, it should say "Come" with a capital "C". This happens because "come again" is a shared text, so when you answer YES and put CANCEL, he'll say "All right then, come again." So that's where that text comes from, however, it is still a mistake.
Other mistake: Get a Pokémon with a Special of 21, and with a move that increases its Attack (as Swords Dance, TM03). Now do the Mew glitch, that is, fly or somehow escape at the same time one trainer sees you by scrolling the screen. Fight with any trainer, then go to Route 13, 14, 15 or 16 and search for a Ditto. Let the Ditto transform into your Pokemon with Special of 21. Now, wait for it to use the move that will increase its attack. It has to use it at least three times. Return to the route where you escaped the trainer. The menu will pop-up, exit and you'll encounter a Mew, however, it will be at least in level 10, and it will know TRANSFORM. Let it transform any of your Pokémon. Then catch it. The game will say you caught a Ditto, not a Mew. The Mew will be gone forever, and instead the Pokémon will be a Ditto.
Add timeDr. James
Other mistake: Marowak's ghost is supposed to be avoided only with the Silph Scope. However if you use a Poke doll in his battle it'll escape and you can proceed the game. This makes no sense whatsoever, because if you do that and advance through the game, you'll find Giovainni with strong Pokemon, but if you go later to get the Silph Scope, Giovainni will be there with weaker Pokemon. Big part of the game can be skipped with this glitch.