Just as the Church doesn’t teach that Prophets are infallible, it also doesn’t teach that Prophets are inerrant. What is the difference? Well, Prophets are not infallible. This means that they are perfectly capable of making mistakes and committing sins. They are also not inerrant. This means that not everything they say is necessarily the directly inspired word of God. In other words, they are capable of making mistakes both in their personal lives and also in their teaching and preaching.
Not everything a Prophet says is inspired by God, even when speaking on religious matters. For some reason, many members of the Church seem to be under the impression that whenever a General Authority speaks about a Gospel topic, or gives a talk in General Conference, or declares his opinion about a particular doctrine, his words may as well be the words of God Himself. This is not true, and I’m sure that President Monson would rebuke anyone who suggested such a thing.
Let me take just one or two examples. It is well-attested to in several reliable historical documents that Joseph Smith publicly taught that the moon was inhabited by people who lived to a very great age and were about 6 feet in height. Now, the beauty of having a living Prophet of God is that, when necessary, they can impart the words of God to humanity. This doesn’t mean that everything they say is the word of God. Joseph Smith was perfectly entitled to believe that there were men living on the moon. In fact, that was a very common belief in the 19th Century. I’m pretty sure that there aren’t any men living on the moon, personally, but in those days it was not such a rare opinion, particularly among less educated people such as Joseph. Was he not perfectly entitled to have personal opinions about these kinds of things? Are we going to say that, simply because of his Prophetic office, he is forbidden to ever express a personal view or speculation about matters such as this one? Never once did he include the idea of men living on the moon in a revelation. Never once did he claim that God had revealed this to him as an inspired doctrine. He was simply expressing his own personal view and speculating, as was quite common in the early 19th Century. Not everything a Prophet says should be taken as if God Himself were speaking through Him. In fact, Joseph Smith himself adamantly declared this to the early Saints. In his own words: “I told [the Saints] that a Prophet was a Prophet only when he was acting as such.”
This same principle can also be applied to things regarding the Gospel. The Restoration is often likened to a glorious dawn of truth, banishing the dark night of error with its golden rays. This metaphor is quite apt, for just as a sunrise doesn’t happen all at once, so the Restoration was (and still is) a gradual process. In fact, it is still not fully complete. The ninth Article of Faith states: “We believe all that God has revealed, all that He does now reveal, and we believe that He will yet reveal many great and important things pertaining to the Kingdom of God.” (emphasis added) As far as I know, this Article of Faith has never been repealed or retracted. It is still in force.
The earliest Saints joined the Church with no knowledge of temple work, eternal marriage, or Priesthood keys. Most of them had joined the Church based on the Book of Mormon alone, without any knowledge of the Doctrine and Covenants or Pearl of Great Price. All of these things were yet to come forth. The morning of the Restoration had dawned, but only partially. In the meantime, awaiting further light and truth, they had to make do with what they had and get on with living the Gospel to the best of their ability. Is it any wonder that they made mistakes? Is it any wonder that, with much revelation and scripture still to be revealed, and only a cursory knowledge of the scriptures which had already come forth, many early Church members and leaders began to speculate about doctrinal matters, even teaching them publicly in many cases and including them in sermons? Yes, there was much taught in the early days of the Church which was pure speculation and was shown to be false by later revelation. We are a Church which believes, proudly, in the necessity of continuing revelation, for precisely this reason. For example, several early General Authorities taught that plural marriage was an eternal requirement of salvation and that it would never be removed from the Church. In fact, Wilford Woodruff himself taught that if the Church ever abandoned plural marriage, it would no longer be God’s church. I imagine he must have felt a little embarrassed after receiving the revelation in 1890 ending the practice of plural marriage. The point is, prior to 1890, it was quite reasonable for church leaders, including President Woodruff, to assume that plural marriage would never be taken from the earth. They could not have foreseen what God had yet to reveal to the Saints. So they taught what they assumed was true, to the best of their knowledge and ability. But in 1890, when God spoke to the President of the Church, Wilford Woodruff himself, the very man who had taught previously that plural marriage was an eternal requirement for salvation, President Woodruff immediately acted upon the new revelation, and published Official Declaration 1, the Manifesto, marking the beginning of the end of the practice of plural marriage in the Church.
There are numerous other examples. Brigham Young, who knew the Bible very well but had not grown up with or become very acquainted with other latter-day scripture, mistakenly believed and taught, quite vigorously, that Adam is God and God is Adam. A close reading of the scriptures quickly reveals that this doctrine is both false and illogical, but it was just one of many various speculations and theories about the nature of the Godhead which were very common in the early Church. It was not until 1916 when Joseph F Smith, President of the Church, who had, possibly, the most thorough knowledge and understanding of the scriptures of any Church president since Joseph Smith, published ‘The Father and the Son: A Doctrinal Exposition’ to clarify and explain the true nature of the Father and the Son as revealed in the scriptures. This put an end to the false theories and speculations which had been so rampant in the Church before. In similar examples, President Joseph Fielding Smith said that man would never land on the moon, and Apostle Franklin D Richards told the Martin and Wiley handcart companies that God would protect them on their journey and they would arrive unscathed. Neither of these predictions came true, because man has indeed landed on the moon and over 200 people from the Martin and Wiley handcart companies died en route. But these cease to be issues once we understand that not everything a Prophet says is the word of God. Often, they are merely expressing their own opinion or speculating. In fact, it is safest to assume that unless they specifically claim to have received a revelation from God, they are just doing their best with the light and truth they have already received. This doesn’t mean we should disregard what they say – they are very wise, good men and God has chosen them to lead this Church for a reason – but it does mean that we shouldn’t be surprised if sometimes they express personal opinions or speculations which later turn out not to be true.