It's difficult to find someone in Church history who was more concerned about details and accuracy than Oliver Cowdery. Here's an example from Letter VIII.
Oliver spent some time discussing the reputation of Joseph and Emma and their respective families. He explained the reason Josiah Stowell hired Joseph to work in the Harmony region in the first place. Note the section in bold below:
Here Oliver explains he is relying on his memory, but he emphasizes "I shall not pledge my veracity for the correctness of the account as I have given."
Previously, in Letter VII, he stated it was "a fact" that the final battles of the Jaredites and Nephites took place in the valley west of the Hill Cumorah.
Those who seek to discredit Oliver are making the argument that when he wrote Letter VII, he was unconcerned with his "veracity" because he stated as a fact something about which he was speculating at best. Somehow, between July 1835 when Letter VII was published, and October 1835 when Letter VIII was published, Oliver suddenly became concerned about his "veracity."
There are other examples, such as this one I've mentioned from Letter IV. He was concerned about his veracity when he noted the "error in the type" in Letter III and corrected it in Letter IV. (Letter III had referred to the 15th year, but it should have been the 17th year.)
In Letter VII itself, Oliver encourages Phelps to "present such facts as are plain and uncontrovertible. [sic]." He distinguishes between fact and speculation several times in these letters, which is why it is so puzzling to have people accept everything Oliver wrote about except the fact that Cumorah is in New York.