Some famous mathematician was to give a keynote speech at a conference.

Some famous mathematician was to give a keynote speech at a
conference. Asked for an advance summary, he said he would present a
proof of Fermat's Last Theorem -- but they should keep it under their
hats. When he arrived, though, he spoke on a much more prosaic
topic. Afterwards the conference organizers asked why he said he'd
talk about the theorem and then didn't. He replied this was his
standard practice, just in case he was killed on the way to the
conference.

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