David, try clearing your cache and seeing what happens. There are a handful (<10) cases of this happening to members and I cannot sort what causes it. The script we use to rewrite URL's was struggling with the First Unread function, as it was a dynamically-created URL. So I modified that template and hard-coded First Unread. Unfortunately, there are still some very isolated instances of it causing problems.

I've tried First Unread in Linus, in Windoze and in OSX, on as many as 5 different computers and two different ISP's and cannot reproduce the error. Which leads me to believe the problem is client-side. Sorry.