Yeah I have the same problem and so do many others. It's a known bug in the BBS and nobody knows where it comes from. Basically you've got a persistent cookie on your machine that corresponds to some settings on the BBS backend which indicate what messages you've viewed. Somehow those settings aren't being saved/restored properly.

It's a bit annoying but it's usually only off by maybe 10 or 20 posts to me. When I see it's happened, I just start reading from the top and going back until I recognize the posts.