When the ball hits the top of the net during a rally and tumbles over to the other side. Unlike a let on serve, a net cord during a rally is a live ball and the point continues. Getting a winner off a net cord is equal parts lucky and infuriating for the opponent. The polite thing to do is raise your hand in apology. Not everyone does.
"Massive rally, then the ball clips the net cord and dies on the other side. She raises her hand in apology. He does not look amused."