If you upgrade SMF from 1.1.2 all the way to 1.1.11 there is a missing chunk of SQL upgrade.  The smf_board_permissions table has permission VARCHAR(10) but there are permissions like "postreplynew" which gets silently shaved into "postreply" due to the column length limitation leaving you tearing your hair out.  (Well it did me.  I was editing templates and everything.)  This results in situations where forum users can no longer reply to posts and trying to edit forum permissions results in database error messages due to duplicate keys ("postreplyown" and "postreplyany" both get shaved into "postreply" for example).

Alter the column permission to VARCHAR(50) and it'll work again.