Example 1:
Compare Alma 51:35
And it came to pass that the army which was sent by Moroni, which was led by a man whose name was Teancum...with Alma 58:16
I caused that Gid, with a small number of men, should secrete himself in the wilderness, and also that Teomner and a small number of men should secrete themselves also in the wilderness.
In the first example, Teancum is mentioned for the first time. In the second example, Teomner is mentioned for the first time. The introduction of Teancum is the norm. We see similar wording when Mormon introduces other people to the narrative. Teomner is the exception. He is not properly introduced. Rather he is mentioned as if the reader knows him already.
There is a simple and logical explanation. Alma 51 is Mormon's abridgement. Alma 58 is a letter from Helaman to Moroni simply copied over to Mormon's plates. Helaman doesn't have to introduce Teomner to Moroni because he is a military leader and Moroni knows him already.
Example 2:
Notice the wording in Omni chapter 1, the end of verse 13 and beginning of verse 14
until they came down into the land which is called the land of Zarahemla. And they discovered a people, who were called the people of Zarahemla.When Amaleki is writing this, Zarahemla still exists but not the people of Zarahemla, because Mosiah became their new king and they merged with the Nephites.
A third example is found here.
These examples make a lot of sense as something written by people who were there to observe the events as they occurred. If they are invented by a poor young farmer, he must have had an incredible sense of detail.