When I was a boy of fourteen, my father was so ignorant I could hardly stand to have the old man around. But when I got to be twenty-one, I was astonished at how much he had learned in seven years.
It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so.
This line is widely circulated under this name. It may be a paraphrase, a later coinage, or a doubtful ascription. It stays because the collection used it; treat the byline as traditional, not proven.
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It ain't what you don't know that gets you into trouble. It's what you know for sure that just ain't so.
Mark Twain
The Commonplace