One day when I was a junior medical student, a very important Boston surgeon visited the school and delivered a great treatise on a large number of patients who had undergone successful operations for vascular reconstruction. At the end of the lecture, a young student at the back of the room timidly asked, "Do you have any controls?"Dr. E. E. Peacock, Jr., University of Arizona College of Medicine; quoted in Medical World News (September 1, 1972), p. 45, as quoted in Tufte's 1974 book Data Analysis for Politics and Policy.)
Well, the great surgeon drew himself up to his full height, hit the desk, and said, "Do you mean did I not operate on half the patients?"
The hall grew very quiet then. The voice at the back of the room very hesitantly replied, "Yes, that's what I had in mind."
Then the visitor's fist really came down as he thundered, "Of course not. That would have doomed half of them to their death."
God, it was quiet then, and one could scarcely hear the small voice ask, "Which half?"
(Quote via Raw Meat, found via Marginal Revolution.)