It's always humbling to think I know a topic pretty well only to meet a true expert on the topic. It makes me think about how much time I spent learning it to not really feel like I know it at all, and makes me second guess my learning process. I can think of several times in my career/life where this has happened.

Also related, learning something, only to find out later that your understanding of something was incorrect, or made incorrect assumptions. Always humbling.