HTTP Error Code

400 The request is malformed, such as if the body does not parse
401 Unauthenticated. When no or invalid authentication details are provided. Also useful to trigger an auth popup if the API is used from a browser
403 User is unauthorized
404 Not Found - When a non-existent resource is requested
405 When an HTTP method is being requested that isn't allowed for the authenticated user.
409 Resource Conflict can be possible in complex systems. It can be caused by fulfilling the request. Duplicate entries, deleting root objects when cascade-delete not supported are a couple of examples.
410 Gone - Indicates that the resource at this end point is no longer available. Useful as a blanket response for old API versions
415 Unsupported Media Type - If incorrect content type was provided as part of the request
422 Unprocessable Entity - Used for validation errors
429 Too Many Requests - When a request is rejected due to rate limiting

500 INTERNAL SERVER ERROR – The general catch-all error when the server-side throws an exception
501, Not Implemented. The server does not support the functionality required to fulfill the request. This is the appropriate response when the server does not recognize the request method and is not capable of supporting it for any resource
503 Service Unavailable. The server is currently unable to handle the request due to a temporary overloading or maintenance of the serve

Operation	HTTP Verb	Response
Update a resource	PUT	200 (OK), with data 204 (NO CONTENT)
Creation of new resource	POST	201 upon successful creation, returning a Location header with a link to the newly created resource You can send the resource body back in response back. You can use alternate attribute (?_body=false) to change the behavior
Read	Get	200 (OK), with data 200 (OK) with empty list - if response is supposed to return collection and has empty data set 404 when specific resource doesn't exist
Deleting a resource	Delete	204 (NO CONTENT) 404 (NOT FOUND) when calling DELETE on a resource a second time