I know this issue is resolved, but I just wanted to add another scenario:
I recently tried to add a favicon to my blog and it caused some havoc on my RSS feeds. I had placed the favicon link (below) in my wp-blog-header.php:
<link rel=”shortcut icon” href=”URL for your favicon”>
and kept getting a similar error everytime you tried to access my RSS feeds. It took me about 45 minutes to figure it out but the validator really helps! Thanks ??