This is rather intermittent, however I periodically see threads that I have read all the posts in and yet they are still marked with the 'new posts' icon.
This could have to do with opening threads in new tabs, though I have not been able to confirm this one way or another. IT is not simply me needing to refresh the page as I have tried that when I catch it happening.
I am rather sure that I have seen the threads marked with the 'no new posts' icon, and then come back latter with them marked as unread again.
<Eternaly_Lost> Me and lolfighter are lovers