infoutils.php (2007-09-30 20:49) line 171 - 172.
if($auth) $info = $auth->getUserData($_SESSION[DOKU_COOKIE]['auth']['user']);
msg('You are part of the groups '.implode($info['grps'],', '),0);
When implementing an auth module that uses trustExternal, it is documented that getUserData() does not need to be implemented. In this case, $info will be invalid, and the implode() call throws an error (causing output to be displayed before headers are sent). Plus the info bar shows that there is no valid authorization module in use (generated from the default getUserData call).
It doesn't appear that getUserData() is used anywhere else in the code when trustExternal is used.