This is apparently a rather slippery problem..it only happens sometimes. Perhaps it isn't happening to you (or at all) at any given moment. But trust me, it's definitely been happening...
Ok: I'm on the main page. I see that there are unread posts on the Troubled Teen forum. I visit the forum, then I click on the unread post. I go back and refresh the forum page, but it's as if I never clicked the link to the post (it shows as unread.) It also shows as unread on the main page.
Again: This problem is definitely intermittent. I have witnessed it as such. It may not happen exactly this way (or at all, for that matter) every time. Hope this helps...
EDIT: And my own posts are even showing up as unread.. something's definitely f'ed with this.