The assumptions that documentation makes about what you already know. Sometimes these are such tacit assumptions that they don’t even give you anything to start googling about. Other times they’re assumptions that you’re already skilled in areas of computing that it can take months to years to understand properly.
It's going to be different for everyone, probably. For me, it was always stuff around networking, systems administration, DB administration, and compiling/linking.
A tiny example might be something that uses a symlink or, worse, is hard-linked from multiple places. A newbie might not even know that such things are possible; and if you don't point it out or use the terms, they can end up be horribly confused.
