As a Physics teacher I never feel truly comfortable that I have figured it all out and that (for want of better words) is a good thing. If I did I would be tempted to sit back and relax which may have its benefits in the short run but from a growth perspective offers dubious value in the long term.
Over the course of the last week I have been working with my Grade Twelves on Special Relativity - a notorious pitfall for conceptual errors. I have taught (not to mention learned and re-learned) the sub-unit over twenty times in my career and I still find oddities that compel me to rethink my understanding of what is arguably one of the most counter-intuitive areas in all of Physics (notwithstanding Quantum Mechanics of course).
You see Special Relativity takes so much of what we formalize in classical physics and turns it on its head. Absolute space - gone, Absolute time - dead, Absolute momentum - see you later, KInetic Energy - we need a rethink. To top this off we teach the topic in a week (due to time constraints - I will avoid the pun) and tantalize the students with such notions as the Twin Paradox and the notion that events that are simultaneous in one frame of reference need not be in another.
Serious stuff - but the kids love it and boy do they challenge me with a series of 'What happens ifs..?' .This in turn makes me hit the books/internet for solutions which in turn improves my own understanding. A wonderful positive feedback loop if ever there was one and for this I am thankful