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phpBB allows post-subjects to be up to 255 characters. Drupal only allows 64 characters (by default).
This causes longer subjects to be truncated during migration -- causing dataloss.
As a minimum migration ought to warn about this before commencing migration of posts -- a simple query will return the length of the longest subject in phpBB:
select max(length(post_subject)) from {posts}
Better yet is to simply alter the {comment}-table to increase the maximum length of the subject, if the above query returns a number above 64.