Description: This error happens when your PHP scripts are sending some content and then trying to modify the HTTP headers. Generally happens when the site is compromised by a script injection or due to an hosting error. Either way, it should be fixed.
Error example:
Warning: session_start() [function.session-start]: Cannot send session cookie - headers already sent by (output started at /public_html/index.php:1) in /public_html/sessions.php on line 101
Clean up: It needs to be verified what is causing the error (hacked or hosting error). Contact support for help at support@sucuri.net.