To Err Is Human; It's Also a Teaching Tool

By DAVID D. PERLMUTTER

Some years ago, after I had filed the grades for a large lecture course, one student -- a loyal attendee and diligent note taker -- dropped by my office to thank me for a great semester. After some pleasantries, she produced a typed note and said, "Oh, by the way, professor, here is a point I caught where you were wrong."

I...