For example, if I go to the Star Wars Fan Edits and Preservation Efforts forum, I see the heading "Announcements", and then as I scroll down there's a big patch of nothing except for the advertisement, and then as soon as I reach the end of the advertisement, the Announcement threads finally appear.
This seems to happen on the forum and sub-forum pages, but not the thread pages.
Since the gap is the same size as the ads, I get the sense that the ads are somehow causing this.