Search system seems to be not handling max $start value for certain search types. Namely, if search_id is set and $start is out of range, user gets a page with no search results but with pagination which depends on the $start value.
Confirmed for search_id=unanswered, search_id=newposts, search_id=unreadposts at least.
Example:

_http://www.phpbb.com/community/search.php?search_id=newposts&start=9999999

(see the screenshot).

Reported by RMcGirr83, clarified by Erik Frèrejean.