The Thunder Slash glitch involves the skill Thunder Slash in Final Fantasy IX. Thunder Slash's base accuracy is set to 0 with a Gravity magic-type effect and as a result, Thunder Slash is almost completely worthless. Though Final fantasy IX has seen various re-releases, the glitch has never been fixed.
There is a second check after this for the target's magic evasion.
There has been various speculation on the bug's origin. A prevailing theory is that Thunder Slash has been given an incorrect Effect code that causes it to deal fractional damage over weapon-based damage, thus instead of using the weapon's Accuracy value, it must use its own Accuracy value, which is 0%. It is possible that Thunder Slash was not supposed to deal fractional damage (19% of the target's max HP), but rather, do x1.9 of normal damage with an added Lightning element.