A mathematician has been invited to speak at a conference. His talk is announced as
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…»