If you're strictly asking about things a self-taught hobbyist programmer may have missed, then I second the suggestions here to skim through a CS degree curriculum and dig into anything that's unfamiliar. As one possible example, maybe you're solving a problem trying to parse some text, and you're in over your head with ad-hoc regexes and conditionals and type casts and exceptions all over the place. If you've seen the concept of a grammar (likely to come up in CS programs, though not all) and of generating parsers / validators for them, you can eliminate a whole lot of programming by specifying a grammar in some common format (for instance, ABNF) and running it through a program that generates a parsing program for you. The general category of programs writing programs is worthwhile to look into, and playing with languages where such a feature is first-class (like Common Lisp, which apart from having macros also has a compile-file function you can invoke at runtime) can be enlightening.
A lot of the comments here are highlighting things that not even most CS degree holders will know about, nor many professional programmers. Those can be useful for a hobbyist too, so maybe the lesson is that regardless of your current level of knowledge it's important to keep in mind: "There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy."
Your comment about feeling comfortable in the practical details reminded me of how I used to feel as a fresh self-taught PHP-wielding teenager after a few years of it. As an "expert" PHP coder, I could do anything! Even an OS if I wanted! Well I learned better eventually. :)
I wondered if someone would ask about that... It's just my own recollection from years ago when I looked at a bunch of CS curricula from public and private schools. There was a lot of patchy variation. If the topics were covered at all they'd typically be done in optional electives like a compilers course, or "theory of computation" course, taken in junior or senior years. Also depending on the school such an elective might not actually be available anymore (I looked at current-semester/last-semester offerings to try and identify those) and is only part of the catalog for historical reasons. Maybe it was offered once, but not anymore, in part because students having the choice would rather take the new data science course or Advanced Networking or whatever to fulfill that elective instead.
I do think the problem is mostly a function of having lots of choice. Everyone's got their own list of what a CS degree absolutely must cover, but there are going to be differences, and schools are incentivized to let students carve their own path. Let's look at this list: http://matt.might.net/articles/what-cs-majors-should-know/ I've looked at a lot of intern resumes over the past few years and broadly they've been impressive from a hire-to-BigCo perspective but I would be surprised if many of them knew much if anything about a large majority of topics from that list. Even narrowing to the "better" schools. I also don't even agree with that list, I just don't think it's realistic to cram all that into a 4 year program on top of all the other STEM courses, humanities courses, and project courses...
Going back further there's the continuing problem of "dumbing down". It ties into a school's incentive to offer more choices to fulfill the graduation requirements (creating an "easy" path), but as a problem in itself it's worth considering. https://www.joelonsoftware.com/2005/12/29/the-perils-of-java... is a classic rant about it, not a very good one but it points at one consequence of the problem. I would bet there's still a sizable bunch of current-year CS graduates who never had to deal with pointers. Maybe I'll do some data exploring some time with the resumes I still have to see what fraction put C or C++ somewhere to make a weak proxy measure.
"they'd typically be done in optional electives like a compilers course, or "theory of computation" course"
Okay, maybe it's because here in Germany, we have a separate line of eduction for "mere" programmers and system administrators as well as a separate branch of higher education for more "practical" skills. But if you study CS at an university, both of these course would be mandatory, together with a lot of math and some physics (to know what actually happens in a circuit).
A lot of the comments here are highlighting things that not even most CS degree holders will know about, nor many professional programmers. Those can be useful for a hobbyist too, so maybe the lesson is that regardless of your current level of knowledge it's important to keep in mind: "There are more things in heaven and earth, Horatio, than are dreamt of in your philosophy."
Your comment about feeling comfortable in the practical details reminded me of how I used to feel as a fresh self-taught PHP-wielding teenager after a few years of it. As an "expert" PHP coder, I could do anything! Even an OS if I wanted! Well I learned better eventually. :)