> Known unknowns: An experienced developer most likely unconsciously knows (has read on HN/Reddit/Book/Blog some time in the last 20 years) that a thing they encounter has been solved already, at least partially.

I feel like my brain is full of "I read a headline about this once" tidbits, in part because of HN. It's like I don't know the answer, but I know the question has been solved before so I know there is an answer.