Favorite Lessons

Anyone remember Mr. Radar? He was one of my favorite teachers - reason is - I clearly remember him saying one day "Scratch that, I don't know what I'm talking about." After that, I trusted anything he said. As an adult I often will repeat that phrase when the situation fits - and think of Mr. Radar when I do.
I don't remember much else that Mr. Radar specifically taught in class though he was a good and fun teacher. But this story shows - we are all teachers as we go about life.
So let's talk; tell about your favorite (direct or indirect)"lesson?"