Issue #1227426 by Mark Theunissen: Fixed variable was removed, but is still being assigned in template_preprocess_maintenance_page().

includes/theme.inc

@@ -2482,7 +2482,6 @@ function template_preprocess_maintenance_page(&$variables) {
   $variables['site_slogan']       = (theme_get_setting('toggle_slogan') ? variable_get('site_slogan', '') : '');
   $variables['tabs']              = '';
   $variables['title']             = drupal_get_title();
-  $variables['closure']           = '';
 
   // Compile a list of classes that are going to be applied to the body element.
   $variables['classes_array'][] = 'in-maintenance';