This turns out to be incredibly true for when I'm tutoring subjects at university. As a tutor you're of course worried a student will stump you with a question from left field but the thing that worries me the most is either telling them the wrong information or mind blanking on something that I should damn well know. As such I spend a lot more time studying the material than I would if I was a student in that course.
My marks going through those courses are all HD territory but if I sat the exam again at the end of tutoring I would have a much better understanding of the content due to the fact that I'm worried if I don't I'll be failing the students I'm teaching.