No MP Sylph spell

From Glitches

From the game Final Fantasy II for the Super Nintendo

Once you have the Sylph spell, simply arrange Rydia’s spell list so that Sylph is in the top-right corner. Whenever you cast the spell, Rydia will not lose any MP. Note, though, that if Rydia does not have at least 25 MP (the normal casting cost), you will not be able to select the spell for casting.

Taken from Gamefaqs