The critical hit bug is present in every version of the original 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.
Explanation of critical rates
A critical rate represents the chance out of 201 that any given hit will be a critical hit. The original Final Fantasy determines critical hits by generating a random number from 0 to 200. If that number is equal to or less than the critical rate of the weapon equipped, the hit is a critical hit.
List of weapons with actual and intended critical rates
Index Number represents actual, bugged Critical Rate.
Critical Rate represents intended Critical Rate.
|Weapon||Index Number||Critical Rate|
Weapon type correlations
An examination of the list reveals a general trend where critical rate was meant to be correlated with weapon type.
|Weapon Type||Critical Rate|
Comparing the intended critical rates with the bugged critical rates reveals that, while critical rates were intended to be reasonably consistent across the course of the game and based on weapon type, they end up increasing linearly over the course of the game, with later-occurring (and generally more powerful) weapons having higher critical rates. Some weapons have critical rates that are up to eight times higher than they were meant to be. As a result, physical attacks are substantially more powerful on average than they would have been had the correct critical rates been used.
Some weapons that were meant to have special advantages have lost their significance because of this bug—most notably the Vorpal Sword and Sasuke's Blade. These weapons were meant to have a high critical rate, six times higher than most swords and daggers. Even though both weapons have higher critical rates in practice, they suffer in terms of relative critical rates, since other weapons get substantially bigger boosts than they do.
With the Vorpal Sword, its uniquely high critical rate would have compensated for its lower Attack Power and Accuracy. Sasuke's Blade would have given the Ninja the advantage of a weapon that generated as many critical hits as the Vorpal Sword, but with much better Attack Power and Accuracy. With the bug in place however, the Vorpal Sword is rendered mediocre and Sasuke's Blade has a much smaller advantage over other high-end weapons.