One of the things they tell us to do in preparation for the job market is to have other people observe our teaching. (Where students don't count as people, clearly. Only colleagues count as people.) This is useful, obviously, for the immediate feedback you can get about how you're doing up there. But it's also good to have practiced being observed so that you don't freak out and lose your shit when you're doing a teaching demo during an on-campus interview. Fair enough.
So this one time a few years ago a colleague of mine was observing my teaching. Debriefing over beers later on, I'd thought the class had gone fairly well. He had a few minor tweaks to suggest. Making sure to call on people from both sides of the classroom. Some tips on how to get that guy to STFU. That sort of thing. Nothing big. But then he said, "Oh, and every time you turn your back to write on the board the entire class can see your thong."