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In Pokémon Red and Green, there are six hidden items that do not function correctly. These hidden items are believed to have been fixed in Japanese Pokémon Blue and later versions (including the localized versions of the Generation I games).
Picking up one of these six specific hidden items causes the other five hidden items to disappear. If the player hands over a Fossil to the scientist in Cinnabar Island's Pokémon Lab (even if they choose not to revive it), all six hidden items reappear.
The broken hidden items cannot be detected by the Itemfinder. Instead, five of these six items have another set of coordinates that the Itemfinder believes them to be located at instead, although there is no item hidden at that position.
These are the hidden items for which this glitch applies.
The actual coordinates and position are where the player can find the item. The specified coordinates and position are where the Itemfinder believes the item to be.
||Rocket Hideout B3F
||One tile right of TM10
||On the eastern machine in the south
||Fence-post to the right of a Trainer Tips sign
||Southeast corner of the area behind a cuttable tree
||Pokémon Mansion B1F
||Northwest corner of the western room with lots of plants
||Copycat's house 2F (Saffron City)
||On the Copycat's bed
||On the Copycat's PC
||Cerulean Cave 1F
||On a rock in the center of the map, accessible from 2F
||One tile to the right
||Cerulean Cave B1F
||On a rock in the northeast
||Outside the map
Pokémon Red, Green, Blue have two coordinate pairs for hidden items in the game data, in the "hidden object" (holds coordinates, text ID, object routine) and "hidden item" (holds maps and item coordinates) databases.
In the hidden object database, all hidden items use the same object routine for hidden items, which determines an item's "picked up" flag depending on how far it appears in the hidden item database.
These six hidden items do not have matching coordinates (one of which is completely missing coordinates in the hidden items database), meaning that "item picked up" flag 255 is used. Hidden item flags start from bit 0 of $D66F, hence flag 255 is bit 7 of $D68E ($D66F+($FF/8)).
$D68E is an out-of-bounds address that is updated whenever the player hands over a Fossil (not necessarily giving it away, but giving it away will work as well) with the identifier of the Fossil, $1F (Old Amber), $2A (Helix Fossil), or $2B (Dome Fossil). None of these items have bit 7 set, meaning that passing over a Fossil will bring back all six items.
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