I have seen it on other boards, nothing to do with deleted posts that I am aware of, more along the lines of a bug with the vBulletin software where when the post count = x it expects there to be a new page so it shows the next page number but when you try to load it defaults back to the top of last page (current) it finds. I have seen some comments indicating that if you change the default # of posts per page to a higher number it happens less frequently.