I think scalability is one of those things.
You might argue that most tutorials actually _mention _it. Well, mentioning it and teaching it is two different things. During my years in web development (which is the last 12 years) I haven’t seen a tutorial that shows you how to make your code scale.
In a successful project in production you will probably soon reach a point where your visitors get too numerous so your application can’t handle it anymore. So you add one more node. And all of a sudden, you get a lot of errors.
Developers should learn about race conditions. Some of these can happen even on a single node, but when multiple nodes are involved, this becomes a real burden. Writing resources from multiple nodes may wreak havoc, and even though a lot of backends (DB, storage, whatever) provide some ways of locking, a lot of developers don’t deal with the locking errors, because they are not taught about it.