I don't have ads enabled here, but I've had a similar thing happen many times on Cubits after I clicked on the "first unread" link. The pattern of behavior on a thread that I'd mostly read already was that after clicking the "first unread" link, it would initially jump to the correct "unread" spot on the page for a second and then suddenly jump back up the page to older posts that I had already read. (I have a slower dsl connection, so someone with a faster connection might not have time to see all the steps of the odd behavior.)
