This notion of “unknown unknowns” resonates strongly with me, and not just on the level of syntax or optimization tricks. My main unknown unknown was the set of all tools that make package writing easier, which (for the most part) you can’t even discover by reading the code because they were used locally by the developers.
I’m trying to address this here:
Reading this discourse is a great way to keep discovering by the way, it is full of extremely knowledgeable people giving answers to questions you never even knew you had. So keep up the good work!