The first mathematics talk I ever gave was a job talk at Brown University on February 1, 1984, and my second talk was in the Harvard Number Theory seminar exactly a week later. It had an all-star cast, including Harvard Professors Barry Mazur and John Tate and Yale Professor Serge Lang.
Right before the talk, I noticed that some of my friends, who were sitting in the front row, were giggling. It was a bit disconcerting when they refused to tell me what they were giggling about; they said they'd tell me afterwards.
I began my talk, and started to write on the blackboard.
Serge Lang screamed, "Stop! Stop! I can't stand it any more!"
I stopped. I had barely said anything. Could I have already said something wrong? What could I have said that would upset Serge Lang so terribly?
"The chalk is screeching on the blackboard!" he explained.
Tate told me to break the chalk in half so it wouldn't screech. I pointed out that all the chalk was tiny---too small to break in two.
We all waited while Barry Mazur ran out of the room, and ran up and down the fifth floor hallway, looking for an open office with chalk.
Eventually, Barry came back with the report "No chalk". What was I to do?
Tate handed me a chalk holder into which I could insert the tiny chalk and use it in a way that wouldn't screech.
Serge calmed down. (He probably went to sleep; I didn't hear from him again.) My talk proceeded uneventfully. But I learned how many Harvard professors it takes to calm down Serge Lang.
What were my friends in the front row giggling about? They were trying to decide whether to tell me that the newly-famous mathematician Gerd Faltings was in my audience. I hadn't yet met or seen him, so I didn't know he was there. That's just as well. There were enough stars to worry about.