LDS Urban Legend: This is how the story goes...after the death of Joseph and Hyrum Smith, the Church was left leaderless. All the members wanted to know who the new leader would be, so a meeting was called. Sidney Rigdon got up first and campaigned for himself to be the next leader. Then it was Brigham Young's turn, and as he spoke, the members witnessed a miracle. It looked to them as if Brigham Young transformed into Joseph Smith before their very eyes. Young looked like Joseph, he sounded like Joseph, and his mannerisms were like Joseph. Clearly this was a miracle to show the members who the next prophet should be.
The only problem is that no one talked about this amazing transformation when it happened. There are no journals, letters, or newspaper accounts written at the time of the meeting that would back up this amazing story. It is true that such a meeting took place. And it is true that both Sidney Rigdon and Brigham Young spoke at the meeting. But no account of the transformation was recorded at that time.
Historian Richard Van Wagoner has searched all diaries, journals, newspapers, and church records written shortly after the meeting and has found no evidence to verify the "miracle transformation" story. You can read more about his findings by reading his published report.
Brigham's speech was reported on in detail in both Nauvoo newspapers and recorded by scribes for the official church records. Hundreds of members present wrote about Brigham's persuasive argument in great detail in their private journals. Nowhere was there a mention of the miraculous or divine.
So where does the story come from? There is no recorded account of the transformation until many years later, after the Saints were settled in Utah. In 1857, 13 years after the speech, Albert Carrington is the first to mention the transformation. In a speech before a huge gathering of Saints, he said that he couldn't tell Brigham from Joseph that day when Brigham was speaking. Soon others were making the same claim.
Records even show that it was impossible for several members who made the claim to have witnessed the miracle to have even been in Nauvoo at the time of the meeting.
But not all members got caught up in the new story. Bishop George Miller, present at the gathering, later recalled that nothing supernatural had occurred on that day. Young made a “long and loud harangue,” Miller later wrote, for which I “could not see any point in the course of his remarks than to overturn Sidney Rigdon's pretensions.”
Richard Van Wagoner wrote probably the best essay on this topic published in Dialogue, A Journal of Mormon Thought, Volume 28, No. 4, Winter 1995.
Van Wagoner details the evolving story showing how the myth of Brigham Young transforming into Joseph Smith was created. Van Wagoner uses the original diaries of the people in attendance at the meeting on August 8, 1844 providing evidence that those there neglected to note the miracle that occurred before their eyes, but later in life the Utah Mormons (including many that weren't even in attendance) describe the transfiguration they supposedly witnessed first hand in great detail. This is a wonderful illustration of how many of the religious myths were/are started.
Another excellent essay on the subject can be found in the Fall 1996 'Journal of Mormon History'. The author is Reid L. Harper and the title is The Mantle of Joseph Smith.