I posted the same issue with TheCartPress and they said the issue has to do with how the plugins initialize. I asked them to connect w/BP developers to look into it. I got the same error again today in my Activity stream when I replied to a post during testing.
Warning: session_start() [function.session-start]: Cannot send session cache limiter – headers already sent (output started at /home/shreemaa/public_html/httpdocs/wordpress/wp-content/themes/shree-maa/activity/comment.php:15) in /home/shreemaa/public_html/httpdocs/wordpress/wp-content/plugins/thecartpress/TheCartPress.class.php on line 781
Part of what I would like to understand – is how clicking a button in the BuddyPress forum to create a group – somehow triggered an error on a PHP page in TheCartPress? How could those two totally different pages/templates/systems trigger that kind of error?