Warning: Cannot modify header information

After activating Directory on a clean WPMU install I received the following error at: searchboro.com

Warning: Cannot modify header information - headers already sent by (output started at /home/boroadmin/public_html/wp-content/themes/twentytwelve/header.php:13) in /home/boroadmn/public_html/wp-includes/pluggable.php on line 876