400 Bad Request

The server cannot or will not process the request due to an apparent client error (e.g., malformed request syntax, too large size, invalid request message framing, or deceptive request routing).

POST, creating entities
400 Bad Request if the entity cannot be created with the information in the request body
PATCH, mutating entities
400 Bad Request, if the modification failed.
Return codes and errors
400 Bad Request: bad syntax (unknown route, missing required fields or parameters, unknown extra parameters, bad field or parameter values).
Those endpoints may return 400 Bad Request if parameters are specified.