Telling the truth when you don't mean to.
Churchill's Commentary on Man:
Man will occasionally stumble over the truth,
but most of the time he will pick himself up and continue on.
“Obviously, a man's judgement cannot be better than the information on which he has based it. Give him the truth and he may still go wrong when he has the chance to be right, but give him no news or present him only with distorted and incomplete data, with ignorant, sloppy or biased reporting, with propaganda and deliberate falsehoods, and you destroy his whole reasoning processes, and make him something less than a man.”
⇒ Arthur Hays Sulzberger