Index: includes/theme.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/theme.inc,v
retrieving revision 1.599
diff -u -p -r1.599 theme.inc
--- includes/theme.inc	7 Jun 2010 06:38:09 -0000	1.599
+++ includes/theme.inc	7 Jun 2010 17:32:05 -0000
@@ -764,7 +764,7 @@ function theme($hook, $variables = array
       }
     }
     if (!isset($hooks[$hook])) {
-      watchdog('theme', 'Theme key "@key" not found.', array('@key' => $hook), WATCHDOG_WARNING);
+      trigger_error(t('Theme key %key not found.', array('%key' => $hook)));
       return '';
     }
   }
@@ -866,6 +866,13 @@ function theme($hook, $variables = array
     if (function_exists($info['function'])) {
       $output = $info['function']($variables);
     }
+    else {
+      // If a function implementation is registered but doesn't exist, it's
+      // either due to a bug in a module or theme, or module or theme code has
+      // been updated without the theme registry being rebuilt.
+      trigger_error(t('Theme function %function not found.', array('%function' => $info['function'])));
+      $output = '';
+    }
   }
   else {
     // Default render function and extension.
@@ -1214,10 +1221,14 @@ function theme_get_setting($setting_name
  *   The output generated by the template.
  */
 function theme_render_template($template_file, $variables) {
-  extract($variables, EXTR_SKIP);               // Extract the variables to a local namespace
-  ob_start();                                   // Start output buffering
-  include DRUPAL_ROOT . '/' . $template_file;   // Include the template file
-  return ob_get_clean();                        // End buffering and return its contents
+  // Extract the variables to a local namespace, setup an output buffer,
+  // include the template file which will output its markup to the buffer, and
+  // return that markup. If the template file does not exist, the "include"
+  // statement will emit a PHP warning, which is desired.
+  extract($variables, EXTR_SKIP);
+  ob_start();
+  include DRUPAL_ROOT . '/' . $template_file;
+  return ob_get_clean();
 }
 
 /**