A few months ago I posted briefly about Sal Khan's academy (here, his link here). He has put up an extraordinary number of talks (1000?) where you see video of what he's "writing" on the screen and Sal talks you through explanations of various topics. It's mostly math, but also some biology, even finance. If you don't know about it, check it out.
There's a lot to like about this project. For starters, Sal knows his linear algebra and calculus, not to mention algebra. Also, it's free. Also, the lectures come as bite-sized chunks which is just right for the modern student with an attention span measured in single-digit minutes if not seconds. One negative aspect is that the system for projecting what he writes is pretty marginal---some kind of a pen would be a lot better.
But the reason I'm re-posting about this is I found a rather egregious error in one of the videos, and it reminded me of my own experience. The error is central to the example (in probability and statistics), in which he explains that "the standard deviation is just the average distance from the average," and proceeds to calculate just as you would if that were true. Of course, it's not true.
Why it's done the way it is, that's a whole 'nother topic.
But that's not my point. This error reminded me of my first teaching experience, long ago. I was a first-year graduate student, and I had barely noticed that I was scheduled to run a "recitation section" for a eukaryotic genetics class. I left my DNA samples on ice and ran to the first meeting of class. Unfortunately, the students expected me to actually explain stuff to them, like what was in the reading assignment on meiosis.
I thought I remembered that. Unfortunately, I remembered wrong. Homologs segregate at division 1, not division 2. Oops.
Almost none of the students in my section turned up for week 2! In fact, the other TA accused me of screwing up on purpose, so she would have more students in her section. (Cindy, I'm sorry, I really am).
Anyway, I did learn my lesson. You must know the material when you walk in the door. And if you don't, be honest (and show up next time with the answer).
I think a similar thing happened to Sal. He has so many videos that he failed to prepare. As James Baker said in his biography: 'prior preparation prevents poor performance.' The five P's. A motto to live by.