From a purely safety-minded perspective, this function has a hidden bound of r[] past which the function becomes unsafe, but it does not check. Not only does it not check, the design of the function means that there is no possible way for it to check safely. s and t are both pointers to characters? How long are the strings they might represent supposed to be? Who knows? This code is incredibly reckless and it's dangerous to introduce students to such carelessly designed examples when first impressions will define how they program for a few years or so.

Ok, so it's reasonable to think that this is supposed to be a teaching example and that considering these concepts might be a bit too early in the process. This leads into the second problem and one that is more subjective: this code is incredibly dense and relies on enough quirks of C that it's almost never going to be clear to a beginner reader what they are supposed to take away from it. It's maybe useful as a quiz question on C syntax and semantics, but there are enough barriers to understanding what the code is supposed to do that the amount of explaining the text would need to do to describe what the code is doing is most likely prohibitively long. Instructive examples should be unambiguous in what they are trying to show, otherwise students will be confused and potentially conflate issues in a way that is difficult to untangle later.

Edit: Ha! I spent so long looking at the first half of the function I totally missed that it was returning r! So, not only does this code have minor issues here and there from its careless implementation, it has a fundamental flaw that, if it were to work, would do so only by accident. I can only imagine that a student might walk away from this example thinking that C functions can return arrays and possibly misunderstand scoping in C.