Yeah, sometimes, though I remember more cases where I was "wait a minute, there's a mistake here!" or "this is completely sub-optimal" and would put that in my notes, and a few paragraphs later he would explain the mistake or how to do it more optimally etc. But I don't see it as wasted effort, more as a small opportunity to pat myself on the back at how well I understand the material - I guess it's the difference between thinking about it five minutes versus half an hour.