It's not the first time either. They hardly ever interact with each other, but when they do, it's always Burr shutting down his "smarty pants" gimmick with very real disdain in his voice. There's a clip from another show they were on together, and Burr gives his opinion on something that Maher disagreed with, and Maher comes back with some snobby vague response about how Burr should "educate" himself more on the topic, and Burr comes back with something like "Oh' really? What books should I read, coach?"
I don't usually buy into these Hollywood beefs, but you can just feel the tension when they're yacking at each. Burr is visibly annoyed and volatile and takes everything as a slight, and Maher is visibly flustered trying to nervously laugh his way through the abuse.
Norm MacDonald ripped on him all the time too. He had this one joke that I'm about to butcher that was so perfect. Something like, "Good news! Every Friday night, there a show that's on where Bill Maher will be giving away the answers to everyone's problems for free!"
He seems to have quite the reputation. Like, it's not an act. He really is that insufferably smug.