Suppose Mustard knows he ought to knock down Green's door if he asks him to (e.g., because Green has locked himself out again and needs Mustard to force the door open), knows he ought to knock down Green’s door if Green is in need of medical attention, and knows that he must not knock down Green’s door if neither of these conditions hold. White often takes it upon herself to try to prevent Mustard from doing what he ought and tries to trick him into doing what he ought not. She’s not bad at doing this, but Plum is even better at doing what she does. She only lets White trick Mustard when she knows that White will trick Mustard in such a way that the deception will not lead him to do what he ought all-things considered not do. (For his part, Mustard knows nothing of these battles between Plum and White.) So, let’s say, White tricks Mustard into believing that Green needs him to knock down Green’s door by forging a note and Plum allows Mustard to see the note only because she also knows that Green needs Mustard to knock down the door in order to give him assistance.
Let’s assume that Mustard’s ignorance of Green’s need for assistance is itself non-culpable as is his mistaken belief that the note gives him reason to knock down the door. Are we to say that he has done anything less than what the reasons required on the occasion? I see no trace of wrongdoing on his part, but the reason for which he acted was not a guiding reason that demanded that he knock down the door. The forged note does not give him that reason and if it were the case that he saw only the forged note and Green did not need his assistance, his knocking down the door would have been all-things considered wrong. He has not shown on his part any sort of disposition to disregard the good or actively pursue the bad. It is not the case that he ought to
have done otherwise.
I've seen people say that it is a principle of practical reason that you should act for an undefeated reason. I don't see that Mustard has managed to do this, yet I see nothing wrongful with Mustard's actions. So, I'm sceptical of the principle. If pressed for an explanation as to why that principle is a principle, I've seen people say that trying to act for guiding reasons is a good way of doing what the reasons require. That might be right, but that seems to suggest that the reason to act for undefeated reasons is an instrumental reason. Doing what that instrumental reason serves as a reason for on the basis of a different reason does not seem to be a way of doing less than the reasons require. So, I guess I'm not sceptical of the principle on a certain reading, but it's not a reading that would lead me to think that facts about the explanatory reasons that moved you would have an impact on the justificatory status of the relevant acts.