This is true for more advanced topics as well. One piece of advice I give people is to not be afraid to re-learn topics. Even getting an A in the class doesn't

This is true for more advanced topics as well. One piece of advice I give people is to not be afraid to re-learn topics. Even getting an A in the class doesn't mean you've really "got" it. I don't think I really "got" linear algebra until maybe halfway through the first year of my PhD.