As a powerful man once said, “There are known knowns… there are known unknowns… [and] there are unknown unknowns… it is the latter category that tend to be the difficult ones.” I’ve never thoroughly understood this "ancient" proverb, but I believe there are all of these things and unknown knowns. That is, things we don’t know that we already know.
For instance, we all know there are offensive players and defensive players on a football field at any given time. Despite this, many of us don’t consider that defensive players affect our fantasy-eligible offensive players on every single play.