When Moodle's built-in web services return an exception, this is often or always returned with an HTTP status 200. For example, if we submit a request to mod_assign_save_grade with an invalid parameter, we receive the response:

Exception: invalid_parameter_exception

Error Code: invalidparameter

Message: Invalid parameter value detected

 
This is returned with an HTTP status code 200 (OK). The 200 code is supposed to indicate that a request succeeded. It is not the appropriate code to use if we passed in bad parameters and got back an exception. A more appropriate code for this example would be 400 (Bad Request).

Use of the correct HTTP status codes would allow our client-side code to more elegantly handle  responses from the server without having to parse every single response for exceptions.