As you use family sources, interviews with Grandma, and stories that were passed down in your family to begin your research, keep in mind that there might be key details that relatives either forgot or intentionally neglected to tell you.
They can be as innocent as forgetting that great-grandpa lived in Idado for ten years and "came back home."
Or they can be intentional, as in forgetting that Grandpa had a wife before he married Grandma and that he had five children with the previous wife.
Omissions can be inconsequential or serious roadblocks to your research. They can also be things Aunt Myrtle simply forgot or something cousin Harold never wanted you to find out.