When a mage or communer gets something up to 100%, it never fails. That should change. How often a learned spell or commune should fail I leave for another debate, but the concept is still true.

Ready ic explanations are infinite, pick your own. An easy one is that no matter how well one knows something, a person makes mistakes. Sometimes you bite your fork when chewing, sometimes you forget the name of someone you've know your whole life, sometimes you just say the wrong word, whatever. It happens because people (whatever race) are not infallible, not machines, not deities (who also make mistakes, I might add).

So every now and then, when a mage casts word of recall, or grease, or soften, or dispel, or anything else, something they have at 100%, they should still fail. Should get a message along the lines of "you mispronounce a word" or "the heat of combat distracts you and you skip a beat in the chant" or whatever.

Spell/commune classes don't do that much more to get 100%s than fighter classes. Indeed, most fighter classes are dumber so they are harder pressed to get there in the first place. And things they work so hard to perfect fail all the time. Which is fine. But it should be consistent for all classes.

Sometimes, the character should just get it wrong and lose the spell.