The Black Swan Fallacy is taught in college logic courses. "I've never seen a black swan," the logic goes, "so black swans don't exist."
The logic is obviously absurd. No one has the opportunity to personally examine the color of every swan on the planet. That doesn't stop us from accidentally falling into this logical trap. We may be very familiar with American swans, for example, and think our experience is sufficient. But black swans actually exist in Australia, if not elsewhere.
There are also people who are very adamant that no two snowflakes look alike, or that there's no such thing as an honest politician. Science tells us the truth about snowflakes. Though I fear the existence of an honest politician is an element of faith one must either have, or not have.
In genealogy we run across this fallacy when one assumes that their search for a particular document has been exhaustive, and thus, the document doesn't exist.
For example, you may find two marriage documents for the same individual, but be unable to find record of an intervening divorce. That doesn't necessarily mean the individual was a bigamist. Before the 20th century divorces were uncommon, and in some places illegal, but even in the 20th century, one doesn't have to file divorce papers in the same county/state where one was married.
Sometimes an ancestor's residence is fairly stationary, narrowing options. However, other ancestors were more mobile. Some might decide that verifying whether everything was legal isn't vital, and focus on other research.
The Genealogical Proof Standard includes a "reasonably exhaustive search." Though we must remember reasonably exhaustive will still miss records. And truly exhaustive is next to impossible. If my maternal grandmother, Myrtle Van Every, hadn't saved a copy of her divorce proceedings, I doubt I would ever have discovered where and when she had married her second husband, Dale Ridgely. They were married and divorced in San Francisco, CA over a period of five months in 1927. There wasn't a single St. Louis City Directory my grandmother missed from 1921-1929. At some point the California marriage records might get indexed on some online database, but I'd have had no reason to look.