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.

Charizard was unable to learn Fly in Red & Blue. This was changed in Yellow which also made it possible to get a Charizard with Fly in Red & Blue via trading.

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.

The three legendary birds (Articuno, Zapdos, and Moltres) are puns on elements, as well as numbers in the Spanish language. Articuno is Arctic (ice) and uno (one), Zapdos is Zap (electricity) and dos (two), and Moltres is Molten (fire) and tres (three). This is also the order that they appear in the National Pokédex.

The original Red and Green Versions were supposed to have a female trainer along with the male trainers, Red and Blue. She was most likely removed due to hardware restrictions, but is still seen on the official strategy guide and artwork. The character was used as the character 'Green' in the comic and 'Leaf' in the remake, FireRed and LeafGreen.

Dratini's sprite as found in Red, Blue, and Green (both the original Japanese release and the updated International release with different sprites) Versions has stripes on its belly. These stripes were removed in Yellow Version and have not reappeared since.

Mew was the first Pokemon name to be trademarked, even before the name "Pocket Monsters".
Mew was trademarked 1990, while Pocket Monsters was trademarked in 1995.

Mew was secretly added at the very end of development and was initially not intended to appear in the game. However, due to a glitch, Mew started to appear seemingly naturally (i.e. without GameShark and the like) in some peoples' games.

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.

On Route 10 in Pokémon Red & Blue, Picnicker Carol gives a description of a chunky, pink Pokémon with a floral pattern. This description perfectly matches the Pokémon Munna, from Pokémon Black & White released twelve years later.

In Copycat's bedroom is a SNES with a game featuring Mario with a bucket on his head. This is a reference to the Japan-exclusive game Mario & Wario, another Nintendo game developed by Game Freak.

During the Professor Oak lecture at the beginning of the game, a Nidorino will be shown as an example of a Pokemon. However, the cry that plays belongs to a Nidorina.

When you encounter your rival in Pokemon Tower, he will ask you why you're in the tower to begin with because you don't have any dead Pokemon to mourn over. This implies that he has lost a Pokemon and has come to pay homage to it or lay it to rest. Furthermore, his Raticate from the previous battle on the S.S. Anne is replaced by an Exeggcute, Growlithe, or Gyrados, depending on the starter Pokemon you chose. This seems to imply that his Raticate somehow died between the battles against him on the S.S. Anne and in Pokemon Tower; common fan speculation states that the Raticate died from wounds received from the battle on the S.S. Anne.

The game contains the now infamous line: "Hi! I like shorts! They're comfy and easy to wear!" The line became a meme among the Pokemon community, and was referenced in Pokemon Black 2 and White 2.

Ninetails was inspired by a powerful, fire-breathing fox with nine tails from Japanese mythology.

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.

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.

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 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: