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(Noun) A large blunder or mistake; an inaccuracy
Today our sociology professor said that dolphins are fish. What a Dewey defeats Truman-she should check her facts.

"Shit! After I printed out my essay, I realized I had made a few punctuation and spelling errors."
"Hey, don't worry. It's no Dewey defeats Truman"
by Arthur Henning September 30, 2011