Bruce R. McConkie teaches that "not every word that a man who is a prophet speaks is a prophetic utterance."
Bruce R. McConkie, "Guidelines to Gospel Study," in Doctrines of the Restoration: Doctrines of the Restoration and Writings of Bruce R. McConkie, ed. Mark L. McConkie (Salt Lake City: Bookcraft, 1989), 231
5. Are all prophetic utterances true? Of course they are! This is what the Lord’s system of teaching is all about. Anything which his servant say when moved upon by the Holy Ghost is scripture, and his command to his ministers is: “The Spirit shall be given unto you by the prayer of faith; and if ye receive not the Spirit ye shall not teach” (D&C 42:14).
But not every word that a man who is a prophet speaks is a prophetic utterance. Joseph Smith taught that a prophet is not always a prophet, only when he is acting as such (Teachings, p. 278). Men who wear the prophetic mangle are still men; they have their own views; and their understanding of gospel truths is dependent upon the study and inspiration that is theirs.
Some prophets—I say respectfully—know more and have greater inspiration than others. Thus, if Brigham Young, who was one of the greatest prophets, said something about Adam which is out of harmony with what is in the Book of Moses and section 78, it is the scripture that prevails. This is one of the reasons we call our scriptures the standard works. They are the standard of judgment and the measuring rod against which all doctrines and views are weighed, and it does not make one particle of difference whose views are involved. The scriptures always take the precedence.