I remembered it happen in a very strange way when I studied calculus. I had a hard time to remember the integration rules and how to apply them. Spent a lot of time on it. But at one point I had a dream where integrals came "flying in" like in a computer game, and I solved them one after another, just like I can solve 1+1 without hesitation. Whether I really solved them or this was just imagined is of course the core question. But the dream was so detailed and crisp, and I felt so conscious in it, that I think I may actually have done that. It was a feeling I very rarely have felt ever before.