Our forum at http://forum.nationstates.net just hit post #16,777,215 and promptly began spewing errors related to the fact that phpBB3's MySQL schema defines post_id (and a few related fields) as "mediumint(8)".

I know we are a larger board than average, but we're not enormous; a built-in limit of 16 million posts seems unduly restrictive.

We manually fixed the problem by converting post_id and similar fields to "int(10)".