Yeah, from what I understand 400 means "fix it at the client's end" and 500 means "fix it at the server's end" which seems to be what the RFC thinks, too:
Basically, 4xx is telling the client you did something wrong (such as visit a non-existent URL), 5xx is when the server errors and can't respond appropriately.
Obvious to you and me. Server team disagreed and tried 422 unprocessable entity, which is not correct. My point is that these issues should be clear enough that there are not thousands of StackOverflow questions on it, and debates between teams. “It’s so easy” is not true. There are layers of understanding.
