I still like the old adage "those who can't, teach." So often I've been disappointed by teachers who try to teach something that they don't know by doing. Just know by reading up on it.
I find that often the "smartest" people don't make the best teachers because they haven't developed the patience that it needs for most of us to grasp a concept. Often the people who have had to learn things the hard way, or try really hard to learn something, make the better teachers.
Case in point -- I watched a Martha S. show where she tryed to saw some wood by holding it with her left hand and crossing her right hand over to saw it!!! It really put her "expertise" in perspective.