Not really anything that can be done about it. Sometimes the cookies just freak out. What you can do though if you have read the threads in the last 24 hours is click on "Get Last Days Posts" on the left side at the top of the forums instead of "View New Posts". It will show you all the posts in the last 24 hours. Then you just stop when you get to the ones you've read. Sorry, but it's the best I can do.
-Steve