error.php

<?php header('HTTP/1.0 500 Internal Server Error'); exit; ?>

index.html

<!--
  There is no way, in Opera, to figure out if an incoming page
  is returning a 304 status.

  Opera provides no indicator as to if a 304 occurred.
   - xhr.status === 0
   - xhr.statusText === ""
   - xhr.getAllResponseHeader() === ""
   - xhr.getResponseHeader( anything ) (not found)
  All of the above is virtually identical to an aborted request in other browsers.

  Example here: http://ejohn.org/files/bugs/304/
-->
<ul id="out"></ul>
<script>
window.onload = function() {
  request("success.php", 200);
  request("error.php", 500);
  request("404.php", 404);
  request("not_modified.php", 304);
  request("wait.php", 0);
};

function request(url, expect) {
  var xhr = window.XMLHttpRequest ? new window.XMLHttpRequest() : new window.ActiveXObject("Microsoft.XMLHTTP");

  xhr.open( "GET", url, true );

  var onreadystatechange = xhr.onreadystatechange = function() {
    // If readyState === 0 then abort occurred, according to Opera
    if ( xhr.readyState === 4 || xhr.readyState === 0 ) {
      // Opera gives status of 0 for 500 errors
      // Unfortunately this leads to ambiguity between errors and 304/Not Modified
      // As not modified pages ALSO return a status of 0
      if ( xhr.status === expect ) {
        log( "PASS: " + url );

      } else {
        log( "FAIL: " + url + " (" + xhr.status + ")" );
      }

      xhr.onreadystatechange = null;
    }
  };

  xhr.send( null );

  if ( /wait/.test( url ) ) {
    setTimeout(function(){
      xhr.abort();

      // Opera doesn't fire onreadystatechange on abort, fire it explicitly
      if ( xhr.onreadystatechange ) {
        onreadystatechange();
      }
    }, 13);
  }
}

function log( msg ) {
  document.getElementById("out").innerHTML += "<li>" + msg + "</li>";
}
</script>

not_modified.php

<?php header('HTTP/1.0 304 Not Modified'); exit; ?>

success.php

yay

wait.php

<?php sleep(10); exit; ?>

A quick Google search turns up this --

http://my.opera.com/community/forums/topic.dml?id=676132

Apparently, a known issue.

I have gotten around a similar Opera bug by outputting a blank space (' ') as the body of my response. Example in PHP:

There is a bug with 304 and empty bodies.

http://www.w3.org/TR/XMLHttpRequest/#response "If the error flag is true return 0 and terminate these steps." that seems to be aggressively enforced on quite a few browsers. Makes it so that only 200s work. The "terminate these steps" is the real problem as the nest step is "Return the HTTP status code"