Cannot modify header informatoin

Here is the warning I received after activating the WPMU DEV Dashboard:

Warning: Cannot modify header information - headers already sent by (output started at /home/content/82/12114282/html/OPD/orthopracticedirectory.com/wp-content/themes/twentytwelve/header.php:13) in /home/content/82/12114282/html/OPD/orthopracticedirectory.com/wp-includes/pluggable.php on line 896

Any idea what triggered the error message. Thanks for your help!