Heh. I did this once. I had a prof in my computer systems class in second-year engineering. He made some comment, I don't even remember what it was anymore -- something about math coprocessors on Intel 80486 chips or something. Whatever it was, it was wrong, and I called him on it. He made some wisecrack that shut me down in front of the whole class (of like 200 people). But I persisted, looked it up when I got home, and emailed him a link. The next day he apologized to me in front of the whole class.
He turned out to be one of my favourite profs!