Make Sure You've Searched All the Records

FamilySearch:

On the home page of https://www.familysearch.org/ click on  "What's New". One article is: "Make Sure You've Searched All the Records" -- "Most people using FamilySearch don't get the full benefit of their search. FamilySearch has 2.34 billion indexed names thast can be searched using the search fields on the home page. If they don't find their ancestor using the search fields they assume they are not in FamilySearch. However, users need to remember that in addition there are 312.4 million names that have not been indexed and cannot be searched using the usual search fields. These collections are referred to as browse only image collections. FamilySearch uses a technique called Waypointing to make these collections available for searching."
Another article is "Understanding Unexpected Search Results" - "Have you ever wondered why particular records show up on your search results when the places don't match? There are a number of reasons for this."