I was using an index to land records that covered the first fifty or so years of the county's land records. One entry was difficult to read. The volume was legible--the page wasn't. Afraid I'd have to go page by page, I viewed the volume and there in the front was an index to just that volume, giving me the actual page number.
The clerk compiled indexes to each volume as they were recorded and years later, a more comprehensive index was created.