Critical Hit bug

The Critical Hit Bug is present in every version of Final Fantasy (though its status as a bug in more recent versions is debatable since it seems to have been intentionally retained). Instead of loading a weapon's critical hit rate, the game loads the weapon's index number. The practical effect of this bug is that critical hits are slightly less common than they were intended to be at the beginning of the game, and substantially more common than they were meant to be at the end of the game.

Along with the Intelligence Bug, the Critical Hit Bug is the most game-changing bug in the original Final Fantasy.