If a forum has several pages of topics and also has (global) announcements, topics which located at the half-forum pages will be duplicated from the page right before the end of the 1st forum half to the page right after.
F.e. if the forum has, say, 2800 pages, topics from the page 1400 will appear at the page 1401, and so on.
The number of duplicated topics is equal to the number of the (global) announcements.
See the screenshots from the .com board.