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.

If the magic and level of Adelbert Steiner (or Beatrix) are high enough compared to the target's level, the skill has a chance to hit, as the hit rate formula is Magic / 4 + Level - TargetLevel.

There is a second check after this for the target's magic evasion.