There is an near-infinite expanse of stuff to learn in every direction you look. Human knowledge and culture is a fractal, even when you think you know everything about a subject there's always more to be surprised by or to do.

I've recently been reading up on cryptography. That part of software engineering alone can take years to catch up with the latest post-quantum developments.

There is also always some basic shit which you forget with time. I need to re-read The Art of Computer Programming at some point.

This isn't specifically related to coding. I'm taking about any manual, be it for a programming language, or a microwave oven. Regarding my particular field of endeavor, how many people that drive cars are even remotely familiar with the actual road rules legislation, or even looked at them since they got their licence? See also the people that complain about following Ikea instructions as if they aren't the simplest possible instructions to follow.