Many spells in the original NES version are bugged:
Aura does nothing at level 8.
Barrier does not grant protection against ice at level 8.
Dispel doesn't work at all.
Protect only works on the caster.
Sap won't work on enemies with more than 256 MP.
The Ultima spell was supposed to increase in power relative to the level of all of the other spells the caster has but won't even if all other learned spells are at level 16. This means that Ultima will only do about 500 damage at level 16.