After enabling javascript_aggregator (no gzip, no JSMin+) I would see the following warning when accessing admin pages (on stock Garland theme). The warning went away after disabling AND uninstalling the module.
warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'phptemplate_closure' was given in /var/www/drupal/includes/theme.inc on line 656.