Videos
Pokemon & Religion - Did You Know Gaming? Feat. Jimmy Whetzel
Pokemon & Science - Did You Know Gaming? Feat. JonTron
Pokemon Part 4 - Did You Know Gaming? Feat. TheJWittz
 
If the player attempts to enter The Safari Zone without sufficient money to enter enough times, the worker who lets you in will take pity and allow you to enter with one safari ball. This was not an interaction in Pokemon Red & Blue and exists as a way to prevent the player from progressing in the game as a result of being unable to acquire enough money to get the HM03, surf, which is found in the park.
Contributed by Psychospacecow
Among the NPC Pokémon trades in the game's data are three trades not assigned to any NPC, and thus inaccessible. One of them, a Beedrill in exchange for a Butterfree, exists in the data of every first-generation title. The nickname of the Beedrill even changed from version to version, but the trade remained unused.

Pokémon Yellow replaced two functioning trades with unused ones. One requests a Pidgeot for a Pidgeot, and the other is a Mew for a Mew. In Japan, both are nicknamed Matsumiya, presumably after Toshinobu Matsumiya, who was in charge of Yellow's scenario. In the English version, the Pidgeot is named MARTY and the Mew is named BART.
Contributed by RadSpyro
In Pokémon Yellow, The Youngster on Route 9 was inspired by the trainer A.J. from episode 8 of the Pokémon anime series. They both use a Sandshrew, speak of striving to win 100 battles in a row, and both Sandshrew know Fissure. In Pokemon Red/Blue, this trainer was originally named Jr. Trainer ♂, used a Growlithe and a Charmander, and never spoke of winning 100 battles.
Contributed by Pogue-Mahone
Attachment
In the Pikachu's Beach minigame, there is an unused 'Radness' bonus of 750 points coded into the game. There appears to be no way to achieve this without hacking.
Contributed by RadSpyro
Utilizing glitches, it's possible to teleport to the the Hall of Fame room directly at the beginning of the game, and beat the game in 0:00 (according to the game's internal clock) with a complete Pokedex. Depending on how the glitch is performed, it can be used to teleport to any location in the game.
Contributed by CosmykTheDolfyn
Attachment
There is a glitch in Pokemon Yellow where you can catch a level 1 Nidoking at the end of Viridian Forest, and level it up to level 100 with one fight. It involves catching a level 4 Caterpies and escaping from a trainer battle using an Escape Rope.
Contributed by EliasAnsel
Attachment
There is a piece of music in the game's code that is not in the final game. It's a song from a previous Game Freak game called Bushi Seiryuden Futari No Yusha and is used in the boss battle of the game.
Contributed by fajitaman
The rival will eventually evolve his Eevee (and thus affect which other Pokemon are in his party) depending on the player's actions early in the game. If the player beats the rival at both Oak's Pokémon Research Laboratory and the early, optional, battle at Route 22, Eevee evolves into Jolteon. If the player loses one of these two battles, or passes up the Route 22 battle, Eevee evolves into Flareon. If the player loses at Oak's Lab, and loses or passes up the Route 22 battle, Eevee evolves into Vaporeon.
Contributed by Funland47
Data exists for a battle situation when the Trainer has no Pokémon, though it isn't actually used when entering a battle with no Pokémon in the final release. In this case, selecting any option other than Run shows the message "Hurry, get away!" This may have been a prototype for the initial battle with the wild Pikachu at the start of the player's adventure, as the player has no Pokémon at that point. It could also have been planned for a possible battle against a wild Spearow with Pikachu unable to battle, making it closer to the events of the anime.
Contributed by Funland47
Attachment
Teaching Pikachu Surf from Pokemon Stadium will unlock a mini-game that can be played in Route 19 by talking to the Surfing Dude in the beach house.
Contributed by Funland47