I've heard the one where the professor is lecturing, and says, "It's obvious that..." A student challenges him: "Come on. Is it really obvious?" The professor looks at the board for a moment, and then runs out of the room. 45 minutes later, the professor returns, and says, "Yes, it's obvious."
“We have a new theorem–that mathematicians can prove only trivial theorems, because every theorem that’s proved is trivial.”
Not very related perhaps but I think this is a great quote:
So I got a great reputation for doing integrals, only because my box of tools was different from everybody else’s, and they had tried all their tools on it before giving the problem to me.
There's a common variation without the student. The prof just pauses after saying "obvious", puts down the chalk, stares at the blackboard, runs out and then comes back, says "it is indeed obvious" and continues the lecture.
