When you are creating web pages that use AJAX you may encounter all sorts of errors. The following list of HTTP response headers might help your debugging efforts :

101 - HTTP/1.1 101 Switching Protocols
200 - HTTP/1.1 200 OK
201 - HTTP/1.1 201 Created
202 - HTTP/1.1 202 Accepted
203 - HTTP/1.1 203 Non-Authoritative Information
204 - HTTP/1.1 204 No Content
205 - HTTP/1.1 205 Reset Content
206 - HTTP/1.1 206 Partial Content
300 - HTTP/1.1 300 Multiple Choices
301 - HTTP/1.1 301 Moved Permanently
302 - HTTP/1.1 302 Found
303 - HTTP/1.1 303 See Other
304 - HTTP/1.1 304 Not Modified
305 - HTTP/1.1 305 Use Proxy
307 - HTTP/1.1 307 Temporary Redirect
400 - HTTP/1.1 400 Bad Request
401 - HTTP/1.1 401 Unauthorized
402 - HTTP/1.1 402 Payment Required
403 - HTTP/1.1 403 Forbidden
404 - HTTP/1.1 404 Not Found
405 - HTTP/1.1 405 Method Not Allowed
406 - HTTP/1.1 406 Not Acceptable
407 - HTTP/1.1 407 Proxy Authentication Required
408 - HTTP/1.1 408 Request Time-out
409 - HTTP/1.1 409 Conflict
410 - HTTP/1.1 410 Gone
411 - HTTP/1.1 411 Length Required
412 - HTTP/1.1 412 Precondition Failed
413 - HTTP/1.1 413 Request Entity Too Large
414 - HTTP/1.1 414 Request-URI Too Large
415 - HTTP/1.1 415 Unsupported Media Type
416 - HTTP/1.1 416 Requested range not satisfiable
417 - HTTP/1.1 417 Expectation Failed
500 - HTTP/1.1 500 Internal Server Error
501 - HTTP/1.1 501 Not Implemented
502 - HTTP/1.1 502 Bad Gateway
503 - HTTP/1.1 503 Service Unavailable
504 - HTTP/1.1 504 Gateway Time-out