Programming logic error
A programming logic error or bug (however, 'bug' may also be used to refer to a glitch in a wider context) in terms of video games is a programming blunder in the game code that prevents the game from doing what it was supposed to do.
An example is the Coin Case glitch in English versions of Pokémon Gold and Silver, where an improper terminator character causes the game program to jump to memory address E112 in echo RAM.
Another example is the Focus Energy glitch in Pokémon Red, Blue, and Yellow, which effectively quarters the player's chances of getting a critical hit, but was changed to increase the chances of a critical hit in other games, such as Pokémon Stadium and Pokémon Gold, Silver and Crystal.