Second type glitch

The Second type glitch (not to be confused with the dual-type Pokémon glitch) is a glitch in and the Japanese Pokémon Blue. It allows the player to transform a Pokémon into another Pokémon equating to the index number of its second type. Without the need of additional glitches, the glitch can be used for obtaining a maximum of 15 unique Pokémon species. More Pokémon are available if the player uses a glitch Pokémon with an invalid second type. The glitch can be used with single-type Pokémon, because internally their "second type" is identical to their first type. It is notable for being one of the first unofficial methods of obtaining in the Generation I, through the use of a pure  Pokémon, most commonly. The glitch predates more popular glitches for obtaining Mew, such as the Mew glitch. The glitch is exploitable through two known methods.

This glitch is part of a division of popular Japanese version exclusive glitches known as select glitches (Japanese: セレクトバグ select bugs), which exploit an oversight with the player's ability to move items using the select button. If the player registers an item using the select button and closes the menu, the position of the select button remains registered. Using this oversight, the player is able to swap invalid Pokémon and moves in the place of other Pokémon by entering another menu without opening the menu via the start button. This is the cause of additional glitches from "swapping" values of the game's.

Effects
The Pokémon that the player receives as a result of this glitch becomes an unstable hybrid Pokémon with a glitchy nickname, with the recipient Pokémon as the original Pokémon, and the donor Pokémon as a Pokémon equating to the index number of the original Pokémon's second type. This means that the Pokémon cannot be traded to a main-series Generation II game. As such, taking the Pokémon into the Day-Care Center on changes it back into the original Pokémon before the player exploited the glitch, although any changes to its nickname and moveset applied after the glitch will remain the same. "Stabilizing" the Pokémon back into its original species (i.e. the recipient) allows the player to obtain a Pokémon with an illegal moveset in a similar way to glitches such as the Pokémon merge glitch. However, the player can stabilize the Pokémon as if it was originally the donor through evolution in the same way that the recipient would, or through.

The glitch has a negative side effect of enabling and disabling flags in the player's Pokédex data and corrupting the player's item pack. The effects on the Pokédex vary depending on the method used.

Available Pokémon through type
Below is a table of the available Pokémon depending on the index number of the original Pokémon's second type. Note that single-type Pokémon have an identical second-type internally.

Through the 13th item
The first discovered method of the glitch involves pressing the select button on the thirteenth item, exiting the menu using B,B and then swapping the thirteenth option with the first Pokémon by using the A button. This is possible either through the use of the Pokémon menu in battle, or the Day-Care Center on. Coincidentally, one of the altered Pokémon flags forces the player to 'see' Pokémon No. 152. A 152nd entry is possible in the Pokédex due to the fact that there are 19 "seen" and "own" Pokédex flags, and a unique Pokémon is stored for every bit. Due to technical limitations, this means that the game must store a minimum of 152 (19*8) Pokémon. In the Japanese versions, this appears as Missingno. (けつばん) but it appears as either or  in the English localizations. The reason for this is unknown.

Through the 14th item
An alternative method changes different Pokédex flags, but does not activate a 152nd entry in the Pokédex. The player must first press the select button on the fourteenth item, exit the menu using B,B and then swap the fourteenth option with the first Pokémon by using the A button through the Day-Care Center. The Pokémon will then turn into an "A" (the Japanese version of 'M (00)) with a glitchy nickname. The player should then ask to receive the "A" from the Day-Care Center, and it will have "grown" to level 1. It should appear in the player's party as the second Pokémon. After swapping the fourteenth item with the first Pokémon through the Day-Care Center again, the player will this time receive a Pokémon depending on the second type of the original Pokémon before exploiting the glitch, while keeping the level 1 "A".

Glitch del secondo tipo