Kent Jaffa created a great genealogy video explaining how Family History research is like being a detective. It's entitled "Is Your Great Uncle Really Your Great Aunt?" In the video Jaffa explains some of the basic rules for good research while tackling a case where good detective work proved that Philander M. Sawyer was really a female and not a male as was shown in the 1850 US Census.
In a follow up video entitled "Advanced Detective Studies" learn why relying on transcripts of records can lead to mistakes, why
you cannot trust a single document by itself and then solve a more
complex problem where children were assigned to the wrong family.