 <p>The error lies on the client side, so you want to use a <a href="http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html#sec10.4" rel="nofollow">4xx</a> status code. I'd go with <a href="http://restpatterns.org/HTTP_Status_Codes/400_-_Bad_Request" rel="nofollow">400 - Bad Request</a>:</p> <blockquote> <p>The request could not be understood by the server due to malformed syntax. The client SHOULD NOT repeat the request without modifications.</p> </blockquote>