In the Woody Allen movie, When In Rome, a newlywed runs into the Clear Only If Known fallacy while looking for a hairdresser. The long list of "turn rights" and "turn lefts" from a well-meaning local is followed by the inevitable “You can’t miss it!”
That last sentence is the hallmark of a Clear Only If Known situation. The speaker assumes prior knowledge on the part of the tourist, information that the tourist just doesn’t have. And it’s why she gets lost.
The Clear Only If Known fallacy is ordinarily benign, a mistake in assumptions. But there is another logical fallacy, not so benign, nicknamed “No True Scotsman.” British philosopher Antony Flew talks about it in his book, Thinking About Thinking.
TVTropes.org defines it this way: A Scotsman reads about a crime committed in England, and thinks, “No Scotsman would do such a thing.” But then something worse happens in a city in Scotland and rather than admit-- even to himself-- that he was wrong about his assumption, he amends his premise: “No True Scotsman would do such a thing.”
The definition of Scotsman has been shifted. The goalpost has been moved. Not only that, the goalpost has been moved from a more objective definition to a highly subjective one.
Abductive reasoning offers another common logical fallacy. Some hikers are walking near a river and find a couple of smooth stones in the grass. Because they found a smooth stone in the river the day before, they assume that the two they found nearby in the grass came from the same river.
But that ignores the fact that there are other rivers nearby, that various kinds of stones look smooth, and that stones, like pollen, sometimes end up far from where they started.
For more about the Clear Only If Known fallacy, try these:
The Clear Only If Known Fallacy is more than miscommunication
Commentary: Without punctuation, sometimes it's all Greek to me
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