FWIW, I have encountered tons of issues in peoples' "this is easy, right?" implementations of 100 Continue (including in some at-the-time major Ruby HTTP servers), non-ASCII encoding in HTTP headers (which has a ton of subtle rules, and just throwing around UTF-8 is not correct), and chunked Transfer-Encoding (though I can't remember what was wrong); further, people often don't even realize HTTP trailers exist at all, which is a bit infuriating :/. (Note, though, that I am still 100% on the side of the anti-HTTP/2 camp in this thread.) The core problem is that somehow people think they can implement a spec without even, you know, reading the spec, which is so far past hubris as to have gone through incompetence into obvious negligence.

Usually the whole 3xx series of return codes, also any 2xx that isn't 200 and 204.

If I find someone's "quick and dirty HTTP client library", I generally lose hope that redirects are handled. Anything more complex than that (and there's a lot!) is pipe dream.