There are many opportunities for Joseph Smith to have slipped up if he had written the Book of Mormon by himself. For example in Alma 17:14 we read:
And assuredly it was great, for they had undertaken to preach the word of God to a wild and a hardened and a ferocious people; a people who delighted in murdering the Nephites, and robbing and plundering them; and their hearts were set upon riches, or upon gold and silver, and precious stones; yet they sought to obtain these things by murdering and plundering, that they might not labor for them with their own hands.
Bruce E. Dale and Brian Dale note how this was an easy opportunity for Joseph Smith to slip up if he wrote the Book of Mormon:
If Joseph Smith “guessed” the Book of Mormon, he would very probably have guessed “precious stones” to be the only precious stones he knew of, namely diamonds, rubies, and perhaps pearls. But Mesoamerica has no rubies at all, nor does it have any significant diamond resources. (Mexico has a few small, inferior diamonds, but no diamond mines.) Joseph Smith would not have “guessed” the precious stones to be jade, obsidian, turquoise or calcite. Nor would the names of those stones have meant anything to all but a very small fraction of those who read the Book of Mormon. (Cureloms and cumoms, anyone?) But Joseph Smith made neither mistake. He (or rather the Book of Mormon authors) simply called them, quite accurately, “precious stones.”– Bruce E. Dale and Brian Dale – Joseph Smith: The World’s Greatest Guesser (A Bayesian Statistical Analysis of Positive and Negative Correspondences between the Book of Mormon and The Maya)
How did Joseph Smith avoid making such an easy mistake?