A doctor amputates the wrong leg. Did he use his medical skill?
Qua performing an amputation, yes, qua amputating the wrong leg, then no. This is the distinction between the secundum quid and simpliciter, where the answer is simpliciter no and yes secundum quid. Understood another way it is the distinction between per se and per accidens, where the answer is per se no and per accidens yes.
The example generalizes to any mistake: I solved the algebra problem for 1=0; you drove into a tree, She baked the pizza crust with plaster of Paris as opposed to flour, etc. If you ignore the modifier (i.e. “into a tree’/ “for 1=0” or “with plaster of Paris…”) then yes you used some skill of solving or driving or baking, but insofar as you include the modifier, you did not.
The example generalizes to moral mistakes if you put the good you were seeking separate from the modifier making the specific difference constituting a sin. In other words, don’t say “I murdered” but, more precisely, e.g. “I sought to preserve my honor by shooting the man at the bar for scuffing my shoes.” Insofar as you were seeking to preserve your honor, there is a real skill and the actualization of some potency, but insofar as you acted in the manner you did, you did not even preserve your honor, but lost your dignity as a rational agent. Again, considered merely as an act of preserving honor, you did a good that even the person you killed could not reasonably keep from you, even though you are entirely condemned for seeking honor in the way you did.
Accounting for mistakes therefore requires distinguishing effects from defects, where an effect is correlative to a cause or actuality while a defect is the absence of that cause or actuality precisely where it belongs. As an effect, the action traces back to pure actuality and divine operation; as a defect, it cannot.
The most interesting defect is the culpable mistake, which demands making the mistake when you know that you’re making it.
