The Thunder Slash Glitch is a glitch involving the skill Thunder Slash in Final Fantasy IX. The result of this bug is due to the fact that Thunder Slash's hit rate was added to Iai Strike's hit rate, resulting in Thunder Slash having a hit rate of 0 while greatly improving the hit rate of Iai Strike. This makes Iai Strike far more potent than it should be and Thunder Slash almost completely worthless, apart from when used by Beatrix; because Beatrix doesn't have Iai Strike in her skillset, she can use Thunder Slash without trouble.
Although Thunder Slash always misses against most enemies, at least Iron Man can be hit consistently.
This glitch has been fixed in at least some of the PAL versions, but is still present in the North American PSN re-release.