This thread: http://www.xsyon.com/forum/showthread.php/7000-5-small-fixes-that-YOU-think-the-game-needs/page2
is bugged for me.
When I hit 'New Posts' I can see the last poster's name. (its Hanover right now). Still, if I open the thread I can only read till page 2, and darkviper1's post appears to be the last one. If I hit 'reply to the thread' and go to the advanced edit mode I can scroll down and can see 3 more posts after darkviper's.

The thread never gets the read status, no matter how many times I open it, it always appears in New Posts.

I logged out of the site then logged back in, and tried it with 3 different browsers, it is the same with all of them.

Same for me. If you do "Mark all as read" it will go away. But still odd. Only thing i can think of maybe the post was deleted by a mod.

That sounds like a problem with vBulletin's thread information. There have been some other problems as well with 4.0.1 but I think they have been fixed with 4.0.2
Might be a mod that didn't correctly update the counter.

You might check the counters: Admin CP -> Maintenance -> Update Counters, first thread and then forum counters/information