I'm sorry if the title isn't the most informative, but this is something that I've had happen to me twice recently. I don't know if it's a problem on my end, I don't know if it's something that really needs to be worried about, but I thought I'd toss it out so that those more knowledgeable than I could take a look at it.
Twice recently, I've apparently had my links history undone. That is, threads that I had visited, which had previously had the 'already visited' link style (non-bolded), were once more in the bolded, 'unread' style. Additionally, the 'New Subscribed Threads' space in my control panel reloaded itself with threads I'd already been to that hadn't been updated since that time. I had not manually logged out at either of these times.
I do not know what caused the problem the first time. The second time, the problem appeared after I hit the refresh button on my browser twice, as the server appeared to be taking too long on the first refresh...that is, the green progress bar at the lower left corner of my Firefox window was visible but empty, and remaining so. I do not know that my double-refresh was what caused the problem, but it is the only thing I can think of that I did about the time the situation occured.
I do not know that this is a critical problem. After all, all that's really happened is that I've regained some old items in the control panel, and I've had to go by memory in figuring out what I've already looked at in the threads. It's not a hardship, it's just something I was curious about. I simply thought I'd toss it out to see if anyone knows anything about why it's happening, or so that any appropriate steps can be taken should they be needful.