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: Specific explanation about 2x2 subtiles and how English R/B (but no other Gen I games) uses the bottom-left subtile for encounter tables but the bottom-right subtile to test if an encounter is allowed on that square. 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). Only after all this, explain how the grass encounter table is not overwritten when entering an area that has no land wild Pokémon, and segue into how the Old man glitch works. Explain why bottom right corner tiles do not share this effect.
Also cleanup for readability.
The English R/B 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, etc. Correspondingly, every space into 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. This is why no encounters are possible on land, warp tiles, or inside a building. Due to a programming error or oversight, when checking which Pokémon is encountered, it uses the bottom-left subtile instead. This is not the case in other Generation I games. Because of this, shore tiles on the western and northern sides of islands cannot have wild Pokémon encounters (as the bottom-right subtile is land), and southern ones are treated as water. However, the eastern ones (called left-facing shore tiles) generate wild encounters, although they are grass encounters. This is because once it is known that an encounter exists, any bottom-left tile that isn't water is treated as grass. The programmers of the games didn't figure this out, and so they'd only load new grass encounters into locations that actually have them, as none are programmed into places like Cinnabar Island and the Seafoam Islands. Through this exploit, however, these can be loaded. Since, during the Old Man's catching tutorial, the player's name is changed to OLD MAN, the player's name is stored into wild encounter data because there is only one possible wild Pokémon during this tutorial and said data is not needed. It is therefore possible to force encounters based on the player's name by visiting one of these locations and surfing up and down the eastern coast. This is the Old man glitch. Another glitch is caused by entering and exiting the Safari Zone to load its encounters into memory and Flying to Cinnabar Island, and then surfing up and down the eastern coast to encounter Safari Zone Pokémon. These can now be battled normally rather than in the Safari Zone style and allows the player to catch rare Pokémon like Chansey.
The glitch is not exclusive to the Safari Zone, 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