This problem has been driving me nuts for a long time, and I've searched and tried everything I've found and still, nothing seems to work.
We use the Post Avatar plugin to display avatars for the member that posts, but whenever we post something new on our blog, we get the avatar we selected, plus this message:
Warning: Cannot modify header information - headers already sent by (output started at /home/lithiumt/public_html/shattered-tranquility.net/wp-content/plugins/post-avatar/gkl-postavatar.php:71) in /home/lithiumt/public_html/shattered-tranquility.net/wp-includes/pluggable.php on line 391
The post still goes through and shows up, but it's damn annoying. It doesn't do it on pages, just on posts.
If anyone has any other ideas to try, we'd be really grateful!