This has happened to me before, and it resolved itself. But it's really bugging me today. Under Forum Tools, when I select "Mark This Forum Read", I have to do it five or six times before it works, and even then, a few minutes later, the same old threads are highlighted as not read. It's not that I'm crossing with new posts; nothing has been posted since I tried it the last time.
It's mostly in Personal Issues (and RI) that this is happening (this is where it happened before), and it isn't because of subforums, either.
It also happens with Feedback