Go to the View Unread Content page. Find a thread which has multiple pages and where the oldest unread post is not in the last page. Click the link which brings you to the oldest unread post. Don't continue to the last page of thread. Reload the View Unread Content page again. The thread does not appear in the list even though you haven't read the last page yet. I run into this sometimes when I don't notice that there's another page. When I click the link again, the thread isn't in the li