Kingler's front-facing sprite erroneously depicts its claws as equal in size, when in reality the right claw is actually much smaller than the left.
Strangely enough, in the original Japanese Red and Green versions, the claws are their correct sizes. The claws are once again their correct sizes in Yellow version.
Additionally, the side that the larger claw appears on was changed after Generation I. The bigger claw became the left claw while the smaller claw became the right.

There's a mistake on Cloyster's sprite. It shows its shell positioned horizontally instead of vertically.

The blurb on the back of the boxes states that 139 different Pokémon can be obtained in each game without trading. This, however, includes all Pokémon that the player must choose between (i.e., both the Omanyte and Kabuto families are counted). Therefore, the true total of different Pokémon obtainable in one adventure without trading is 124, which excludes the two unchosen starter families, one Fossil family, two Eeveelutions, one of Hitmonlee and Hitmonchan, and the four Pokémon that only evolve when traded.

The old man, who is laid down protesting his lack of a morning coffee in the localized version, is actually passed out from drunkenness in the Japanese version. You would have to bring him coffee in order to sober him up.

Red and Blue's default names in the game's coding are Ninten and Sony, respectively, a reference to Nintendo's rivalry with Sony in the gaming industry, i.e. the Playstation vs. the N64.

There is a glitch where the player can fish in large statues found in gyms and Indigo Plateau, even being able to catch Pokemon. You can find Magikarps with the Old Rod, Poliwags and Goldeens with the Good Rod, and Krabbies, Psyducks, and Goldeens with the Super Rod. The Super Rod only works in the Cerulean City gym. You can also surf on these large statues, but no Pokemon will appear while surfing.

There were originally going to be 190 Pokemon instead of 151; 39 of them were later moved to Pokemon Gold and Silver. By checking the game's code, you can see the orders in which the Pokemon were entered into the game, Rhydon being first and ending with Victreebel. Since Ken Sugimori, the designer of the Pokemon, confirmed that Rhydon was the first Pokemon ever designed, it can also be inferred that the Pokemon were put into the game in the order in which they were originally intended to appear in the Pokedex. The ordering also proves that all 190 Pokemon were conceived before the original games' release.

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.

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.

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.

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.