Taught a concept in class incorrectly then fixed his mistake. When I studied the material after learning it the way he explained and found the correct information though other material, I emailed the professor. He wanted to meet me after our next class. This was initially to show me that I was wrong and to clear up my misunderstanding. He saw where I was going and we went to the lab to test it. We proved that I was right, admitted he had tought it the wrong way for years and said he would change his lesson. The next class, he returned to the subject and correctly explained the concept. Later I worked on a big research project with him as my advisor. He expected a lot and was tough, but we had mutual respect and that made the work fun.