Riemann Jokes

A mathematician has spent years trying to prove the Riemann hypothesis - without success. Finally, he decides to sell his soul to the devil in exchange for a proof. The devil promises to deliver a proof within four weeks.
Four weeks pass, but nothing happens. Half a year later, the devil shows up again - in a rather gloomy mood.
"I'm sorry", he says. "I couldn't prove the Riemann hypothesis either. But" - and his face lightens up - "I think I found a really interesting lemma..."

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..."