If you had been indexing the 1820 census for Nicholas County, Kentucky, how would you have read the last name of the Sarah who appears in this image?
I know what it is (or at least think I do), because of research I have done on this person in this location. But an indexer usually has not done extensive research on all the families in a certain area. Consequently they must guess as to what the last name is.
For reasons like this, one always has to think of other ways letters could be read and other ways sounds could be spelled.