In 1984 my supervisor at work, John, was also my next door neighbor. He was black, and married to a white woman. We became friends, and he would often ask about my Mormon religion (As we were know back in the day!). As was inevitable, soon he asked about blacks not being able to hold the priesthood before 1978. I, being the the good returned missionary with all my knowledge and wisdom, told him the reason blacks could not hold the priesthood before 1978 was because they were descendants of Cain, and were less valiant before they came to earth. This is what I had been taught in church. This made sense to me.
My friend told me that was the stupidest thing he had ever heard of, and that he would never join the Mormon church. I just took pity on him 34 years ago, but today I agree with him, that it is one of the stupidest things I’ve ever said! And guess what, the LDS church agrees with me!
Today, the Church disavows the theories advanced in the past that black skin is a sign of divine disfavor or curse, or that it reflects actions in a premortal life; that mixed-race marriages are a sin; or that blacks or people of any other race or ethnicity are inferior in any way to anyone else. Church leaders today unequivocally condemn all racism, past and present, in any form 
I so wish I could go back and change what I said to John. I wish I could find him today and apologize all these years later for what I said. If I had a do over, I would tell him we don’t know why blacks could not hold the priesthood, that I was uncomfortable with the policy, and that I’m glad they changed it.
I’m not the only one that has put my foot in my mouth. From a book written in 2000 by two BYU religion professors:
“David Whitmer maintained the prophet used an oval-shaped, chocolate-colored seer stone slightly larger than an egg… Such an explanation is, in our judgement, simply fiction created for the purpose of demeaning Joseph Smith and to undermine the validity of the revelations he received after translation the Book of Mormon” (Ostler and McConkie, Revelations of the Restoration, Deseret Book, 2000: 89-98)
From the Book of Mormon Translation essay 15 years later:
The other instrument, which Joseph Smith discovered in the ground years before he retrieved the gold plates, was a small oval stone, or “seer stone.” As a young man during the 1820s, Joseph Smith, like others in his day, used a seer stone to look for lost objects and buried treasure. As Joseph grew to understand his prophetic calling, he learned that he could use this stone for the higher purpose of translating scripture.
So what dumb things have you said about church doctrine/policy/history that you wish you could take back, and has now been repudiated with the church’s own Gospel Topics?
 Gospel Topics: Race and the Priesthood