From Bulbapedia, the community-driven Pokémon encyclopedia.
During the planning and development stages of Pokémon FireRed and LeafGreen, several parts were going to be a part of the games. Some of them were dropped or changed, but some of those dropped concepts can still be found in the game's programming.
The islands in the Sevii Islands region would likely be named Sevii Isles. In addition, several more locations in the Sevii Islands have been found in the code, including Sevii Isles 6-9 and 22-24. Sevii Isles 8 and 9 have fully coded map data, implying that they were scrapped very late in the game's development.
A records room like that in the Hoenn games has also been found, but cannot be accessed without the use of a cheating device.
A map of the interior of an unused house in Routes 19 and 23 has been found. It is shared between the two routes and the usage for them is unknown. However, it is possible that the one in Route 19 is related to an originally planned comeback of the Pikachu's Beach minigame from Pokémon Yellow.
There are also other unused maps in the programming of FireRed and LeafGreen. Only few of them are complete and there are some that appear to be alternate versions of the maps in the final release. However, their purpose is unknown.
In addition to these maps, 18 more have been found. However, 17 of them don't have defined data and the other has an invalid pointer to it.
Key items exclusive to Pokémon Ruby, Sapphire, and Emerald are found within the programming code, complete with item sprites, and are obtainable only by hacking. For the most part, they have no use, though the Mach Bike and Acro Bike will act the same as the normal Bicycle obtainable in Pokémon FireRed and LeafGreen with the exception of not being recognized by the game for entering Cycling Road. Strangely, all of these items have accurate bag descriptions that are different from those in the Hoenn-based games (which have a different menu setup and less room for the description).
HM08, unobtainable except by cheating and containing Dive like in the Hoenn games, can be taught and deleted freely, making its use almost the same as a normal TM.
In the localized releases of FireRed and LeafGreen, the text of dialogues is colored depending on the person's gender: blue for males and red for females, while other text remains in the usual black. In the original Japanese versions, no such distinction is made, thus all text is colored black. However, in pre-release footage from the games, this distinction is present even in the Japanese releases. The reason for this change is unknown. The video below, related to an early sprite for Professor Oak, shows this for a split second, at 34 seconds of playback.
| The beta sprite for Professor Oak
There is a beta sprite for Professor Oak that would have been used in the introductory part of the games. The only time it has been seen was on footage from one of the Japanese commercials, of which can be watched below.
| This video is not available on Bulbapedia; instead, you can watch the video on YouTube here.
Unused overworld sprites of Mew, Raikou, Entei, Suicune, Celebi and Deoxys' Attack and Defense Formes have also been discovered in the coding. Their use is not known but it is possible that they might have been planned for events.
Leftover weather effects
The same weather effects from Pokémon Ruby and Sapphire are present in the coding of FireRed and LeafGreen and are listed below. Furthermore, an unused effect present in the coding of all Generation III games is present, which consists of three snowflakes falling.