We've seen this in every debate going back many years now. Someone takes a potshot trying to get the opponent to lose their cool and fumble a bit. It's usually phrased in a somewhat innocuous way rather than an outright attack, usually somewhere in the center though ("you're no Jack Kennedy", a famous practiced line). However it doesn't always work well, the most you get is laughs from the audience but the opponent picks up and carries on. It really doesn't provide any new information about a candidate's political positions or views except to show how catty they are.
This time around though... the standard goading actually worked as candidate A loses his composure and forgets all the practiced "stay cool" advice. Now candidate B's campaign knows exactly where the red buttons are to push during the next two elections. And they get pushed again, and again. Even the most novice of political candidates usually trains how to do a debate properly, how to give a speech that sounds like he's got a highschool diploma, and so on.