I'm sure it bothered him, too. Keep in mind that the goals of university courses are not just to learn the material but to learn how to learn something. I know it bothers faculty members when they barely feel ahead, but at the same time -- teaching well is as much about understanding pedagogy as it is about understanding the content. And there's a lot of computer-science specific teaching techniques that would transfer to this course.

In an intro course, you seldom get into the super advanced parts of that topic. He's still a very distinguished CS instructor, and even if a student were to ask some question about, say, a CSS cascade question, he'd be more than capable of looking up the answer and explaining things, even if he doesn't know something on the spot.

I'll add that when live coding, you need to generally be more than just comfortable explaining a topic -- even if you understand say, using jQuery callbacks with AJAX requests, you'd probably avoid teaching it live until you know how to recover from errors in reasonable ways or exactly where in the docs you need to look. It's not that you couldn't figure out the solution, but you want to be comfortable debugging something on the order of seconds of to a minute or two so you don't loose lecture time.