Status code 304 in cached error pages

When Drupal serves an error page from cache, it returns 304 as status code if headers if-modified-since and if-none-match match expected values, as defined in includes/bootstrap.inc

function drupal_serve_page_from_cache(stdClass $cache) {
  // ...

  // See if the client has provided the required HTTP headers.
  $if_modified_since = isset($_SERVER['HTTP_IF_MODIFIED_SINCE']) ? strtotime($_SERVER['HTTP_IF_MODIFIED_SINCE']) : FALSE;
  $if_none_match = isset($_SERVER['HTTP_IF_NONE_MATCH']) ? stripslashes($_SERVER['HTTP_IF_NONE_MATCH']) : FALSE;

  if ($if_modified_since && $if_none_match
      && $if_none_match == $etag // etag must match
      && $if_modified_since == $cache->created) {  // if-modified-since must match
    header($_SERVER['SERVER_PROTOCOL'] . ' 304 Not Modified');
    drupal_send_headers($default_headers);
    return;
  }

  // ...

From W3 specifications about If-None-Match header:

If the request would, without the If-None-Match header field, result in anything other than a 2xx or 304 status, then the If-None-Match header MUST be ignored.

So cached status code must be checked, and this block must be skipped for 4xx status codes.