Mostly in jest:

    "I think there is a world market for maybe five computers."
    -- Thomas Watson, chairman of IBM, 1943

    "There is no reason anyone would want a computer in their home."
    -- Ken Olson, president, chairman and founder of Digital Equipment Corp., 1977

    "The concept is interesting and well-formed, but in order to earn better 
    than a 'C,' the idea must be feasible."
    -- A Yale University management professor in response to Fred Smith's paper 
       proposing reliable overnight delivery service. 
       (Smith went on to found Federal Express Corp.)

    "Heavier-than-air flying machines are impossible."
    -- Lord Kelvin, president, Royal Society, 1895.

    "Stocks have reached what looks like a permanently high plateau."
    -- Irving Fisher, Professor of Economics, Yale University, 1929.

    "Louis Pasteur's theory of germs is ridiculous fiction".
    -- Pierre Pachet, Professor of Physiology at Toulouse, 1872

    "640K ought to be enough for anybody."
    -- Bill Gates, 1981

    "The bomb will never go off. I speak as an expert in explosives." 
    -- Admiral William Leahy, U.S. Atomic Bomb Project.

I did come across an interesting letter that is not necessarily relevant to this discussion but might be worth reading:

http://chem.tufts.edu/answersinscience/relativityofwrong.htm