In the Pokémon games, there are several error codes which can appear when the game is not functioning properly. These error codes are likely used internally to identify what the problem with the game is.
In Pokémon Red and Blue, there are several error codes that appear when the game is not functioning correctly. Problems such as these sometimes crop up when the game's memory access is corrupted and can occasionally appear when experimenting with glitches such as the Mew glitch. Usually, when an error code appears, the game does not freeze and the player is allowed to continue playing the game without resetting. Currently, there are 15 known error codes in the Generation I Pokémon games:
Text that appears on screen
Possible reason for error
The text that is currently in the text box is wrong and should normally be in a different place. Sometimes this error causes strange or glitchy characters to appear.
The game is unable to complete an event. For example, this error message can appear when the player is talking to someone and the game does not have anything valid to say. This can be seen in Pokémon Yellow when the player finishes the game with 152 entries in the Pokédex.
This error can occur when the game confuses battle events with regular events; sometimes when the player switches Pokémon beyond the sixth slot or incorrectly performs the Dokokashira door glitch, this can lead the player into a fight with a glitch Pokémon, or freeze the game.
There is an error displaying in-game text when reading objects such as signs and TVs.
The file data is destroyed!
This error can appear when the player has damaged the game data to a point of no repair, sometimes through the use of glitches which can alter the game save, or when a cross-language trade is attempted.
The current position of the player character is unrecognized. This can cause strange or glitchy characters to appear. 
The game is unable to display the correct/required text.
This error message can appear when a game attempts to perform an event but the Pokémon in the player's party is unrecognized. For example, when the player attempts to trade a non-hybrid glitch Pokémon with a non-playable character.
The sound clip that is playing is unrecognized, this can sometimes play random Pokémon cries.
When this error occurs, the screen will turn white and the words "48 ERROR" pop up before quickly reverting back to whatever the player was previously doing. This error can be caused by the glitch Pokémon a for unknown reasons.
The game is unable to display the correct/required text. The glitch Pokémon'Ng'Mp's Pokédex entry triggers this error.
This error is similar to a 5 Error, however it has also been seen after text such as "This is my graThe trainer blocked the ball! I2"
This error can occur when data transferred between two cartridges via link cable is damaged, corrupted, or otherwise glitched.
This error can occur when data transferred between the Game Boy Tower and the Game Boy game is damaged, corrupted, or otherwise glitched by the Transfer Pak.
Sometimes the game will seem to mix error codes in between text; for example "9" with 2 Error. This would display "92 Error." but this is not an official error code.
A 0 Error in Pokémon Red
A 4 Error in Pokémon Red
A 5 Error in Pokémon Red
A 6 Error in Pokémon Red
A 56 Error in Pokémon Blue
i hate to edit a page, but i have encountered a 10 error code in pokemon yellow with a unnamed glitch trainer using a gameshark code.
Pokémon Gold, Silver, and Crystal feature a more detailed method of handling errors compared to Red, Blue and Yellow, although error messages are uncommon in-game without the use of a cheating device.
This text appears when the player attempts to talk to a character who normally cannot be talked to and thus has no talk event defined. 
'No windows avail-able for popping.'
This text appears when an event attempts to bring up a message window yet the specified text/function is undefined - notably appears when HM06 is used outside of the TM/HM pocket 
'The window save area was exceeded.'
Theoretically would appear when too many overlapping popup windows are displayed. 
It is currently unknown when this may appear. A possible error regarding an event which has invalid data. 
Theoretically would appear when the player triggers a hidden event (such as leaving New Bark Town without a Pokémon) which has no script defined. 
'The save file is corrupted!'
This error message commonly appears when the player attempts to select Continue but the game has no valid data to load. For example, the game may give this message when data has been corrupted by a cross-region trade. This may also occur if the internal battery inside the game cartridge is dying or has died.