Index: cacherouter/engines/file.php
===================================================================
--- cacherouter/engines/file.php	(revision 2179)
+++ cacherouter/engines/file.php	(working copy)
@@ -99,8 +99,12 @@
         fclose($fp);
       }
       else {
-        // t() may not be loaded
-        drupal_set_message(strtr('Cache write error, failed to open file "%file"', array('%file' => $file)), 'error');
+        if (user_access('administer site')) {
+          // t() might not be loaded
+          drupal_set_message(strtr('Cache write error, failed to open file "%file"', array('%file' => $file)), 'error');
+        }
+        // We can assume watchdog is available since we we're in the 'set' function and to get here they should have a db?
+        watchdog('cache', 'Cache write error, failed to open file "%file"', array('%file' => $file), WATCHDOG_WARNING);
       }
     }
   }