I am receiving the following error in my system.log file:
2011-01-12T14:16:52+00:00 DEBUG (7): HEADERS ALREADY SENT: [0] C:\\xampp\\htdocs\\www.mysite.com
I see this too. I think it has something to do with images in WYSIWYG. Try watching the logs whilst going through the admin (especially CMS pages) and you might see it happen. It's harmless.