When clicking unread posts in the menu, you can further search in these topics using the 'search this forum...'. When I then enter a search term and click search I then get an overview of all the post containing this search term.
I can then use the pagination to go to the next page. The problem is that the phpBB says there are for example 50 pages (depending on the search term) while I can only click on the first few pages (again depending on how often the search term is found. For example, for a given search term I can only go to page 4 when phpBB says there are 38 pages. Clicking on page 5 does nothing and only loads the last page (in this case page 4).
I can reproduce this on area51 forum.
I don't know if you understand what I mean, but to try for yourself would be to go to unread posts and do a search. Next click on the last page of the pagination. You will then see that that doesn't work.