This will happen if the discussions module is on your default page, it will only happen when you first navigate to the site and click on the discussion module. You will notice that when it happens you will not have ?tabId=1 or something similar in the address bar. The bug is entered into the bug tracker and I found several ways to fix it, but at the expense of breaking something else. I have another idea of an approach, but I've been fighting viruses all week and haven't had a chance to check it out.
Bruce