D&C 132

As priesthood leaders are lay members of the Church and are not necessarily perfect, the Lord provides some importance counsel concerning the Prophet Joseph Smith where He instructed -

"...if a man be called of my Father, as was Aaron, by mine own voice, and by the voice of him that sent me, and I have endowed him with the keys of the power of this priesthood, if he do anything in my name, and according to my law and by my word, he will not commit sin, and I will justify him."  (
Doctrine and Covenants | Section 132:59)

In my mind, the Lord is basically saying that if a priesthood leader is called and set apart, even receiving priesthood keys, and in his calling he does things in the name of the Lord according to the laws and words of the Savior, having not committed any serious sin, the Lord will support and justify his counsel.  That is why the Lord follows with -

"Let no one, therefore, set on my servant Joseph; for I will justify him; for he shall do the sacrifice which I require at his hands for his transgressions, saith the Lord your God."  (Doctrine and Covenants | Section 132:60)

In other words, having followed this pattern committing sin which is not serious, the Prophet is still a man that must repent of his sins.  Even so, the Lord will justify him in his calling.