This bug is repeatable.
If I leave one of the forum threads open in my browser and go away for a few hours, then come back and refresh the page, The little green crosses that indicate that there are new posts disappear completely. It's like I am logged in as a guest.
If I click on the 'Account' link in the main page menu and open my account page, then navigate back to the forums, the problem disappears.
Using WinXP and IE8
