During development, a type called "Bird" was going to be in the game, but was later changed to flying. This is still in the game's code, with Missingno. being a bird/normal type.

Pokemon Red & Blue were originally supposed to allow players to use badges as an item. When used outside of battle, a looped version of one of the jingles played after Oak evaluates the player's pokedex progress will play. Only the Boulder Badge and Cascade Badge were usable in battle and were able to throw a rock or bait at a Pokemon, like in the Safari Zone.

Concept art shows a shop owner with a Pokemon on a shelf. This may indicate that Pokemon were originally going to be sold in shops.

In Pokémon Red, Blue, and Yellow, as well as their remakes FireRed and LeafGreen, a truck was placed in the S.S. Anne's port. The truck seems to serve no purpose. Its presence in the remakes could be a reference to the original rumors that Mew could be found under the truck.

In the Pewter Museum of Science in Pewter City there is an attraction featuring a space shuttle. In Red & Blue, the plaque accompanying the space shuttle model reads "Space Shuttle Columbia"; however, in the Generation III remakes the plaque simply reads "Space Shuttle". This is probably due to the fact that on February 1, 2003, the real Space Shuttle Columbia disintegrated upon re-entry during its 28th mission, killing all seven crew members. The reference to Space Shuttle Columbia was kept in the Japanese version of the Generation III games.

There is an exploit that, when used correctly, allows your Ditto to trick a wild Pokemon into using Transform.

This is only possible with the Pokemon Pidgey, Pidgeotto, Spearow, or Fearow as they are the only wild Pokemon who know Mirror Move. If your Ditto uses Transform on a wild Pokemon who uses Mirror Move in the same turn, The wild Pokemon transforms into Ditto. The game also registers the wild Pokemon as Ditto.

In the Kanto region, all of the towns are named after colors, the only exception being Pallet Town, which was named after the word "palette," or a range of colors used by an artist or in a picture.

There is a man who is located at Cinnabar Island's Pokemon Laboratory who requests a Raichu in order to trade for an Electrode. After the trade is complete, the man then says 'The RAICHU you traded to me went and evolved!'

This is a translation error where the Japanese version of Pokemon Blue has a Machoke in place of Raichu where as Pokemon Red and Green have Raichu.

In the game's code there is a trainer class for "Chief" called "シルフのチーフ" which translates to Silph's Chief. This possibly meant that the player had to battle Silph Co.'s president.

The Pokémon sprites were changed in Japan for their special edition of the game, Pokémon Blue. Originally, Pokémon Red and Green had different sprites to those of the English release. These new sprites were used in the international release of the game. It's widely agreed that the revised version in Pokémon Red & Blue are far superior to those in Pokémon Green, where a lot of proportions were out of place.

There is an unused item written in the code of the first Pokemon games that would allow the player to ride on water without having a Pokemon use Surf. It has been given the fan name "Surfboard"

The Japanese names of Vaporeon, Jolteon, and Flareon are literally Showers, Thunders, and Booster.

Koffing's sprite is wrong. The artist drew the skull and bones above its face rather than below.

The hotel in Celedon City has an invisible PC in the corner. If you stand one space down from the top right corner of the first room and press A, you'll be able to boot it up. It's assumed that this is a slight oversight, as the hotel resembles a Pokemon center, and the programmers removed the PC graphic but not its function.

There is evidence to suggest that Ditto is a failed clone of Mew. Both Pokemon are genderless, weigh the same, their base stats are exactly the same in every respect, they share almost identical colors, and they're the only two Pokemon who learn Transform naturally.

Ditto can also be found commonly in the same cave as Mewtwo, as well as the mansion on Cinnabar Island, where experiments on Mew were said to take place.

There's a battle against Professor Oak programmed into the game, but was never included in the final game. It is only accessible by using a Gameshark or glitching the game.

His team consists of: lv.66 Tauros, lv.67 Exeggutor, lv.68 Arcanine, lv.70 Gyarados and the starter Pokémon that neither you or your rival chose at the beginning of the game, fully evolved at lv.69.

According to a journal entry in the game, Mewtwo's birthday is February 6th.

In a 1997 interview with Famimaga 64, Satoshi Tajiri said that the Pokémon Porygon was made due to people saying that he should develop a 3D game instead of a sprite-based game on the Game Boy:

In an interview with Junichi Masuda, he stated that the source data for the game was almost lost because the computer that stored the data suffered from a nasty crash.

In the Pokemon Origins anime miniseries, Red uses a Charizard, Dodrio, Persian, Scyther, Lapras and Jolteon during his fight against Blue at the Indigo Pleateau. These were the same Pokemon on the team used by creator Satoshi Tajiri during his test playthrough of Pokemon Red & Blue.