--- includes/menu.inc	2010-12-23 14:04:30.000000000 +0100
+++ includes/menu.inc	2010-12-23 14:11:22.000000000 +0100
@@ -492,6 +492,19 @@ function menu_execute_active_handler($pa
       menu_rebuild();
     }
     if ($router_item = menu_get_item($path)) {
+      // If we are on a valid page and the server has responded with a 404
+      // it means that the request is being handled by the 404 handler and
+      // the site will not work correctly.
+      // Turn clean URLs off, if they are on, and then redirect the user to
+      // the page they were expecting.
+      if (isset($_SERVER['REDIRECT_STATUS']) && $_SERVER['REDIRECT_STATUS'] == 404) {
+        if (variable_get('clean_url', '0')) {
+          watchdog('system', 'System no longer supports clean URLs, turning off automatically.', array(), WATCHDOG_WARNING);
+          variable_set('clean_url', 0);
+        }
+        drupal_goto($read_only_path, array(), 307);
+      }
+
       if ($router_item['access']) {
         if ($router_item['include_file']) {
           require_once DRUPAL_ROOT . '/' . $router_item['include_file'];