Ok, so professors are never accurately or semi-accurately played on TV. They are usually lechs. This week, on How I Met Your Mother, one of my favorites, Ted is starting as a new architect prof at Columbia. I felt immense pain as he screwed up his first twenty minutes or so of his first day. But then, it didn't matter because he was in the wrong room, so he could start fresh with his real class.
Nope, I never lectured to the wrong group of students. However, on my very first day of teaching, at the U of Vermont where I was a visiting professor (one year deal that became a two year deal), I forgot to bring my syllabi to the first class. And then I repeated the mistake later that day when I was teaching my second course (same topic, Intro to IR). And I did it again (or not did it again) when I was teaching my third class that day (Intro to IR--same topic to three different sets of students).
So, I was an idiot my first day. I think a more realistic one than portrayed this week, though.
The painful lecture is about 2:30 into the video below: