In the Japanese versions of Pokémon Red, Green, and Blue, you could get stuck in an infinite loop with the Old Man (Video):

1. Have a full party and every box on the PC full
2. Ask the Old Man to teach you how to catch Pokémon
3. The Old Man goes to throw a Pokéball at the Weedle
4. The game will say that you have a full box and can't catch any more Pokémon
5. Steps 3 and 4 repeat forever and the only way to get out of it is to reset your game

The reason for this is described at Bulbapedia

This is because the game thinks that the player is catching a Pokémon, even though it's actually the Old Man. The game produces a "temporary" Pokémon to hold, and if the catching demonstration ends, that caught Pokémon is deleted.

This was fixed in the Japanese version of Pokémon Yellow and all non-Japanese versions of the game.

You can get in a never-ending loop with the Old Man (Japanese Red, Green, and Blue)

In the Japanese versions of Pokémon Red, Green, and Blue, you could get stuck in an infinite loop with the Old Man (Video):

1. Have a full party and every box on the PC full
2. Ask the Old Man to teach you how to catch Pokémon
3. The Old Man goes to throw a Pokéball at the Weedle
4. The game will say that you have a full box and can't catch any more Pokémon
5. Steps 3 and 4 repeat forever and the only way to get out of it is to reset your game

The reason for this is described at Bulbapedia

This is because the game thinks that the player is catching a Pokémon, even though it's actually the Old Man. The game produces a "temporary" Pokémon to hold, and if the catching demonstration ends, that caught Pokémon is deleted.

This was fixed in the Japanese version of Pokémon Yellow and all non-Japanese versions of the game.

In the Japanese versions of Pokémon Red, Green, and Blue, you could get stuck in an infinite loop with the Old Man (Video):

1. Have a full party and every box on the PC full
2. Ask the Old Man to teach you how to catch Pokémon
3. The Old Man goes to throw a Pokéball at the Weedle
4. The game will say that you have a full box and can't catch any more Pokémon
5. Steps 3 and 4 repeat forever and the only way to get out of it is to reset your game

The reason for this is described at Bublapedia

This is because the game thinks that the player is catching a Pokémon, even though it's actually the Old Man. The game produces a "temporary" Pokémon to hold, and if the catching demonstration ends, that caught Pokémon is deleted.

This was fixed in the Japanese version of Pokémon Yellow and all non-Japanese versions of the game.

In the Japanese versions of Pokémon Red, Green, and Blue, you could get stuck in an infinite loop with the Old Man (Video):

1. Have a full party and every box on the PC full
2. Ask the Old Man to teach you how to catch Pokémon
3. The Old Man goes to throw a Pokéball at the Weedle
4. The game will say that you have a full box and can't catch any more Pokémon
5. Steps 3 and 4 repeat forever and the only way to get out of it is to reset your game

The reason for this is described at Bulbapedia

This is because the game thinks that the player is catching a Pokémon, even though it's actually the Old Man. The game produces a "temporary" Pokémon to hold, and if the catching demonstration ends, that caught Pokémon is deleted.

This was fixed in the Japanese version of Pokémon Yellow and all non-Japanese versions of the game.