This has happened to me in the past if I logged in to the forum, and then for some reason while reading new posts had to leave the computer for a couple of hours. In that situation, I think their is a timeout function in the forum software, where if you don't do anything on the forum for a certain amount of time it will drop all those unread posts. That one is a little annoying, because I know when I log in that I have to have enough time to read all the new posts, lest I "lose" them from the unread posts page and have to search down the main page for them individually. The same thing has happened to me on the "Show new replies" page as well.