A certain Greek professor, let's call him AF, happened to have attended medical school in the US before becoming a professional mathematician.
He attended a talk by another mathematician, who claimed to have proved in N dimensions a result which AF had struggled to prove for N=2. Disconcerted, he spent the entirety of the talk constructing a counterexample to the speaker's result.
At the end of the talk, when questions were invited, AF walked up to the board and wrote down his counterexample. He turned around as he heard a loud thump from behind him. The speaker had fainted.
Undeterred, AF used his medical training to revive the speaker before returning to his seat.