When a mage or communer gets something up to 100%, it never
fails.

You just don't lose concentration. That is.
Yes. When your spell is perfected you don't lose concentration.
Nothing more.

But will it have any affect on the target depends on many things.
And it is nowhere near "always working".
Some spells always have affect when casted, like detects or word of recall.
The same is for skills. There are skills that always work when perfected.
For example: 100% flurry never fails. or we should change it?
Some of spells inflict partial affect ( damage of spells may be reduced because of saves ).
Some of spells hardly work even when perfected.

Overall, we have similar picture for skills.
Some of them always work when perfect, others don't.


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.

You can fail to cast a perfected spell.
For example if you under affect of immolation.