This error can be fixed by viewing the armor screen from the pause menu. If they have no weapon equipped, but do have armor on, the game will mistakenly set their defense to their level, as if they were not equipping any armor. There exists a bug in the NES version, where upon level up, the game checks if the Monk/Master has a weapon equipped instead of their armor to determine what formula to use. Several weapons are also bugged in the NES version, in which the weapons' intended special effects do not take effect. The spells intended to raise effectiveness of physical attacks and evasion are glitched in the NES version, as is the item House, which saves the player's progress before it restores their spell charges. The critical hit bug has been retained through various remakes, but the Intelligence bug has been fixed in recent releases. The Intelligence stat is broken, meaning Red Mages are just as powerful spell-casters as White and Black Mages.
As a result, physical attacks are substantially more powerful on average than they would have been had the correct critical rates been used.
The Intelligence bug and the critical hit bug are the most game-changing bugs.