--- includes/theme.inc.orig	2010-05-29 01:27:07.000000000 +0200
+++ includes/theme.inc	2010-05-30 12:37:50.000000000 +0200
@@ -866,6 +866,13 @@ function theme($hook, $variables = array
     if (function_exists($info['function'])) {
       $output = $info['function']($variables);
     }
+    else {
+      $output = '';
+      trigger_error(
+        t('Theme function does not exist: @function', array('@function' => $info['function'])),
+        E_USER_NOTICE
+      );
+    }
   }
   else {
     // Default render function and extension.