I'm using a somewhat old plugin (it hasn't been updated since '09) called Popularity Contest to show the popularity of my posts. However, I noticed that it was breaking my RSS feed. This is due to the fact that the plugin doesn't properly escape the ampersand character inside an image URL. Instead of & it uses just &. You can fix this by changind line number 2272 to:
$str .= '<img src="'.site_url('?ak_action=api_record_view&id='.$post->ID.'&type=feed').'" alt="" />';
