Drowzee's sprite has five fingers. It is supposed to have three.

It's possible to initiate a glitch that allows the player to walk through walls by use of the Safari Zone's limited steps, and having a poisoned Pokémon. By having the game think that you're still within the Safari zone while outside, and jumping over a ledge on the 500th step, the game will send the player back to the entrance of the Safari Zone while still having it within its data that they are currently mid-jump. When exiting a building with this glitch active, the game will crash, but it's possible to exit the entrance room of the Safari Zone by having the player's Pokémon die of poison. Using this glitch allows the player to skip certain sections of the game, but is riddled with a number of limitations.

The 7th gym leader originally had a different design. Because of its similarities to early artwork to Blaine, as well as his appearance in the anime, this design is likely the original design for Blaine.

Ken Sugimori revealed on his Twitter that Brock was originally meant to be the second Gym Leader, with what looks like a Youngster being the original first Gym Leader.

Promotional material for the game lists Paras with the name "Parasyte". This name may have been changed in order to avoid copyright issues with the 1990 manga series, Parasyte.

In an interview with Yomiuri Online, Pokemon designer Atsuko Nishida revealed that the reason why Pikachu was made a rare encounter in Viridian Forest was because of her own personal favoritism towards the character and what she described as a feeling of "not wanting to let other people have Pikachu."

Indigo Plateau's background music sounds very similar to the Mother 1 (EarthBound Beginnings) Mt. Itoi theme.

The international versions of Red & Blue are based on the Japanese version of Blue. Many changes were made in this version, including updated graphics and sound, as well as several bugfixes found in the Japanese versions of Red & Green.

After Shigeru Miyamoto became Satoshi Tajiri's mentor, Satoshi gave thanks to him by making the default rival name 'Shigeru'. Satoshi made himself the default name for the protagonist. This can be found in the Japanese Generation I Pokemon games.

Some Pokemon share the same cries. Some examples are Charizard and Rhyhorn, Ditto and Poliwag, and Goldeen and Caterpie.

On Cinnabar Island if you don't have the key to the gym with you there is a small glitch you can perform. First, use Surf on the right hand side of the island and move upwards until you're across from the door. Get off your Pokemon and walk until you're in front of the gym door where you'll get the message about it being locked. Look up and you'll see an old man on the roof while this text is displayed.

The Spanish translation of the first generation Pokémon games is known for being very poor quality with many translation errors. One of the most remarkable mistakes is the translation of the move "Slam" into "Portazo", which actually means "Door Slam". This name remains even in the latest games (5th Generation).

The article "the" is used when announcing trainers for battle (for example, Charlie the Bug Catcher. It worked for typical trainers but not Gym Leaders, Elite Four, your rival, or link battles. A screenshot in the instruction manual of the English Red and Blue still contains the text "The Brock wants to fight!", possibly suggesting a late removal. The instruction manual also calls them Pokemon Leaders, instead of Gym Leaders.

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.

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

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.