[dekhn]

There's lots of stuff I knew well and then forgot, but can re-learn quickly. For example, nearly all of calculus (useful when dealing with machine learning). Other bits I've retained and never forgotten, such as everything I've seen involving matrices. There are even things which I had conveniently completely "forgotten" but later emerged as suppressed latent memories- for example, set theory. I was so unhappy with the lead-up to Russell's paradox that I actively suppressed thinking about sets, groups, rings, and fields for several decades.

There are even other bits that I was shown, never incorporated into my brain at all, but later recognized as truly important (Taylor series expansions, the central limit theorem, the prime number theorem, etc).