Therefore, he had Mosiah brought before him; and these are the words which he spake unto him, saying: My son, I would that ye should make a proclamation throughout all this land among all this , or the people of Zarahemla, and the people of Mosiah who dwell in the land, that thereby they may be gathered together (Mosiah 1:10)This may seem vary strange without the background given in the Book of Omni. The king of Zarahemla is called Benjamin. He wants his son, Mosiah, to make a proclamation to the people. He calls them the people of Zarahemla and the people of Mosiah. Why? Mosiah has not started reigning yet and it would be weird to talk to Mosiah and ask him to send a proclamation to the people of Mosiah. Why not "your people"?
The Book of Omni clears things up because it tells about the whole background for the land of Zarahemla.
So we learn that Mosiah was Benjamin's father and that he discovered the land of Zarahemla. "The people of Zarahemla" and "the people of Mosiah" in Mosiah 1 is therefore consistent with Omni and helps us realize that the Mosiah passage quoted refers to King Benjamin's father, not his son. Having read the Book of Mormon several times, we know this already. But that is only because we read Omni right before we read Mosiah. When Joseph Smith just starts dictating from Mosiah, he just jumps right into all this without the background that Omni provides. After having dictated about 600 pages of text 2.5 months later, he gets to Omni and gives the background, including this subtle detail about "the people of Mosiah" into it.12 ...Behold, I will speak unto you somewhat concerning , who was made king over the13 ...and they were led by the power of his , through the wilderness until they came down into the land which is called the .14 And they discovered a , who were called the people of Zarahemla. Now, there was great rejoicing among the people of Zarahemla; and also Zarahemla did rejoice exceedingly, because the Lord had sent the people of Mosiah with the which contained the record of the Jews...19 And it came to pass that the people of Zarahemla, and of Mosiah, did together; and was appointed to be their king...23 Behold, I, Amaleki, was born in the days of Mosiah; and I have lived to see his death; and , son, reigneth in his stead.
This detail is just one out of many in a long and growing list of evidences that Joseph Smith simply received a text from God that had already been carefully crafted.