I'd say most web APIs I've used or developed recently return JSON even with 4xx or 5xx error codes. What can be annoying is knowing what JSON schema to parse with depending on the status code, as not even the Content-Type header will tell you that. APIs (esp. those behind load balancers) that sometimes return HTML and sometimes JSON are far too common though - the problem there is that the JSON responses are appropriate for programmatic consumption (terse/semantically well defined) where HTML ones typically aren't. But even if the Content-Type header is abused (application/octet-stream anyone?) it's not hard to write code that copes with either. One API I used recently returned JSON in some cases and CSV format in others, with no distinction between status code OR content type!