There's an edge case where thread read marking isn't quite intuitive. If you read a thread to the end, then it's marked as "read". Then, if you go back to the first page of it, and post a quick reply, your reply is added be you are not taken to the last page of the thread. Now go back to the containing forum, and the thread is now marked as "unread", but your post is the last post there.
Since you have technically read your own post (it was inserted into the page, after all), the thread should not be marked as containing unread posts.