From Bulbapedia, the community-driven Pokémon encyclopedia.
The Fight Safari Zone Pokémon trick is an officially-acknowledged glitch in Pokémon Red and Blue. It is performed by entering and exiting the Safari Zone, then going to Route 20 without traveling through any area that contains wild Pokémon on land, and finally Surfing along the east coast of any island on the route. This is most commonly done by Flying from Fuchsia City directly to Cinnabar Island, then Surfing on the island's east coastline which is considered part of Route 20. The glitch can also be performed using the east coast of the Seafoam Islands, as there are no land wild Pokémon on Route 19 in between.
When performed, the wild Pokémon that appear while Surfing will not be the usual water encounters for Route 20, but rather will be wild Pokémon from the Safari Zone area the player was most recently in. This glitch thus allows players to battle and catch normally Safari Zone-exclusive Pokémon as if they were regular wild Pokémon, bypassing the Safari Zone's mechanics, hence its name.
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Reason: Link to explanation on Smogon forums. Explain how this leads to the Viridian Forest/Safari Zone encounters oversight where certain grass tiles will never give wild encounters (this oversight is currently undocumented on Bulbapedia).
The maps in Red and Blue are made up of groups of tiles, each of which are composed of 2x2 subtiles. For example, the player takes up four subtiles: one of them is the player's top left, another is the top right, and so on. Correspondingly, every tile onto which the player can move is made of four subtiles. When the game is checking if an encounter is possible it uses the bottom-right subtile. An encounter is only allowed if this subtile is grass or water. Due to a programming error or oversight, when checking which Pokémon is encountered, the game uses the bottom-left subtile instead. However, this is not the case in other Generation I games. While choosing the Pokémon to encounter, the game assumes that non-water subtiles are grass tiles.
Each boundary tile between land and water contains both land and water subtiles. Because of this, shore tiles on the western and northern sides of islands cannot have wild Pokémon encounters, as the bottom-right subtile is non-grass land, while the southern shore tiles are treated as water. However, the eastern tiles (called left-facing shore tiles) have a bottom-right water subtile and a bottom-left land subtile. While Surfing on these tiles, the player therefore encounters Pokémon as though on land. However, the game only updates the grass encounter list when entering an area where there are grass encounters. If the player uses this trick in places without grass encounters, like Cinnabar Island and the Seafoam Islands, they encounter Pokémon from the last area they visited with grass. The player can thus enter and exit the Safari Zone to load its encounters, fly to Cinnabar Island, and surf along the eastern coast to encounter Safari Zone Pokémon.
The glitch is not exclusive to Safari Zone encounters, but can in fact be performed using any area. The wild Pokémon that appear on the east coast of Cinnabar Island and Seafoam Islands will simply be the wild Pokémon that appear in grass in the area the player was last in that has grass.
Old Man glitch
- Main article: Old man glitch
During the Old Man's catching tutorial, the player's name is overwritten with OLD MAN. Because there are no wild Pokémon in Viridian City, the player's name is stored in the grass Pokémon encounter list. Using this glitch allows the player to encounter wild Pokémon based on the characters in their name, which can be any Pokémon in the game, potentially over level 100. Depending on their name, the player can also encounter the glitch Pokémon MissingNo. and 'M (00), as well as Glitch Trainers.