The marking of threads as read or unread does not work correctly. I haven't figured it out exactly yet but some threads with new unread posts seem to get marked as read. Sometimes threads have been marked as read even though I havenít even visited them at all.
I guess there is some type of timestamp check and maybe my local time zone (CET) compared to server time screw things up. But as totally unread threads also have this problem there must be something more.