Aside from countless spelling and grammar corrections across the years, I was apparently the first person to mention to my (well-seasoned) precalculus teacher last year (10th grade) that the Cartesian plane was developed by Descartes. He thought the Cartesians were famous for their maps or something. Most of my teachers have been knowledgeable about their chosen subjects, but I've been correcting teachers about facts beyond their expertise for about eight years now.