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Two closed menu Select glitches are known to have occurred in the Nintendo Space World 1997 trial demos of Pokémon Gold and Silver. These glitches were documented 20 years later after these ROMs were leaked online, but likely were known prior to the leak.
Both of these Select glitches allow for the player to swap out of bounds Pokémon in the party or moves in battle.
While the 'long' method of the closed menu Select glitch which occurs in Pokémon Red, Green, and Pokémon Blue (Japanese) (involving entering a battle in between pressing the Select glitch) does not work in the 1997 trial demo, two other methods are possible in the demo including one which worked in the v1.0 release of Pokémon Red and Green.
Item use method
This method only allows access of invalid Pokémon (or a regular Pokémon switch).
1. Press Select on an item in the inventory (e.g. the second item).
2. Scroll to Potion (キズぐすり) or Full Heal (なんでもなおし) and press A to open the Pokémon menu.
3. Press A to open the party. The cursor should be still active as selected. Pressing A again allows the player to swap the selected Pokémon based on the last cursor position.
If the player only has one Pokémon, using the second item allows for the player to obtain a hybrid between glitch Pokémon 0xFF (ボゃマﾞ０ミﾞ) and glitch Pokémon 0x00. Using the third-fifth item allows for the player a stable glitch Pokémon 0x00.
This glitch also occurs in the v1.0 release of Pokémon Red and Green.
1. Enter battle.
2. Open the items menu and press Select on an item.
3. Close the items menu
4. Either open the party menu and press A to swap a Pokémon in battle, or open the moves list of the active Pokémon and press Select to swap it.
This glitch can lead to the partial switch glitch, access of glitch Pokémon (as above) or access to glitch moves. If the cursor was +1 or more of the number of moves in the list, it is possible to obtain a new move or the hex:00 glitch move if the user's Pokémon has three or less moves and the cursor was on (number of moves+1).