Have noticed this when I'm browsing before I log in, probably because that is 20 posts/page while I have mine set to 100. If a thread spans more than one page and you advance to the next page using the numbers at the bottom [1][2][3] then page 2 starts at 21, page 3 starts at 42, page 4 starts at 63, etc. If you use the number bar at the top it seems fine.