A mathematician has been invited to speak at a conference.

A mathematician has been invited to speak at a conference. His talk is announced as
<p>Proof of the Riemann hypothesis.

When the conference actually takes place, he speaks about something completely different.

After his talk, a colleague asks him: "Did you find an error in your proof?"

He replies: "No - I never had one."

"But why did you make this announcement?"

"That's my standard precaution - in case I die on my way to the conference..."

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