For the last 24 hours or so, at the top of this website it says "Unread (1)" but when I click on that link it tells me "Your unread topics appear here" and yet there are no unread topics even though it said (1). I haven't seen this behaviour before.

It's a minor problem. Just a little annoying because I used to like it when the number (1) used to actually mean "1 unread topic". nothing serious, but is there a reason for this new behaviour?

I've seen that before too. I don't know why it shows up occasionally, but manually refreshing the page with R has always made it go away for me.