line 28, column 0: Use of unknown namespace: com-wordpress:feed-additions:1 (11 occurrences)
Yup. Was thinking something like <site xmlns="com-wordpress:feed-additions:1">1234</site> inside the rss/channel element for an RSS feed and the feed element of an Atom feed. The namespace here is something I just made up, so feel free to use something more sane.
<site xmlns="com-wordpress:feed-additions:1">105219076</site> <item>
<post-id xmlns="com-wordpress:feed-additions:1">45686</post-id> </item>