Following error was received during an upgrade from 7.21 to 7.22:

Notice: Undefined variable: site_name in include() (line 25 of /homepages/26/d110147064/htdocs/drupal7/sites/all/themes/metropolis/maintenance-page.tpl.php).

This was in a multi-site configuration (i.e. site in specific location in the sites directory but theme installed in master code base)