I've just learned a valuable lesson about Ancestry.com census records.
I was searching for a particular Coffey family in 1840 North Carolina using the Ancestry census index. The index points to them in Wake Co.
That particular census (according to Ancestry) consists of 172 images. The family I was looking for, along with other families known to have been related, was found on image 144. The only problem is that the enumeration for Wake Co. ended at image 105 with Yancey beginning at image 106.
I was finally abe to figure out the problem, thanks to a sharp-eyed fact checker who questioned why this particular Coffey family would be in Wake Co. at that time.