Most common failure codes
400 Bad Request
The request could not be understood by the server due to malformed syntax. The client should not repeat the request without modifications.
The request requires user authentication. The response must include a WWW-Authenticate header field containing a challenge applicable to the requested resource. The client may repeat the request with a suitable Authorization header field. If the request already included Authorization credentials, then the 401 response indicates that authorization has been refused for those credentials. If the 401 response contains the same challenge as the prior response, and the user agent has already attempted authentication at least once, then the user should be presented with the entity that was specified in the response, because that entity might include relevant diagnostic information.
The server understood the request, but is refusing to fulfil it. Authorization will not help, and the request should not be repeated. If the request method was not HEAD and the server wishes to make public why the request has not been fulfilled, it should describe the reason for the refusal in the entity. If the server does not wish to make this information available to the client, the status code 404 (Not Found) can be used instead.
404 Not Found
The server has not found anything matching the Request-URI. No indication is given of whether the condition is temporary or permanent. The 410 (Gone) status code should be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address. This status code is commonly used when the server does not wish to reveal exactly why the request has been refused, or when no other response is applicable.
500 Internal Server Error
The server encountered an unexpected condition which prevented it from fulfilling the request.
501 Not Implemented
The server does not support the functionality required to fulfil 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 server. The implication is that this is a temporary condition which will be alleviated after some delay. If known, the length of the delay may be indicated in a Retry-After header.
504 Gateway Time-out
504 Gateway Timeout happens when the server, while acting as a gateway or proxy, did not receive a timely response from the upstream server specified by the URI (such as HTTP, FTP, or LDAP) or some other auxiliary server (such as DNS) it needed to access in attempting to complete the request.