The Route 24/25 theme has a strong resemblance to the Sky Theme from Mario & Wario, a Japanese-only Super Nintendo game which was also developed by Game Freak.

The infamous Missingno glitch can be triggered by the player having Pokemon knowing the moves "Fly" and "Surf", going to Viridian City and triggering the Old Man's catching tutorial. Afterwards the player must Fly immediately to Cinnabar Island and surf along its eastern coastline without moving away from it or back onto the island itself, eventually triggering an encounter with Missingno (Or a similar glitch Pokemon named 'M), after which the item in the player's 6th Item slot will be duplicated to a maximum of 255.

This glitch is triggered by the game's internal data replacing the player's chosen name with the "Old Man" string for the sake of the tutorial and it not being properly replaced by another battle. Whatever letters are used in the player's name will also change what form Missingno will take. For example, a W being used as the 3rd, 5th or 7th letter will trigger the Kabutops Skeleton Form Missingno, an X will trigger the Aerodactyl Skeleton Form and a Y will trigger its Lavender Tower Ghost Form.

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.

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.

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.

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.

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."

In 2020, an unreleased extended cut of the 1998 Pokémon Generation I bus commercial was uploaded to YouTube. This extended cut shows that the bus featured in the advert was actually crushed during production, despite the final commercial cutting from the compactor starting to the compactor having already crushed the bus. The unreleased crushing scenes do not have Pokémon superimposed into the windows.

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.

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

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.

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.

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

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

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 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.

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.

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 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 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.