The phpBB3 code makes the assumption that no non-trvial bbcodes will be added through mods (or to the main codebase).

There are 0+12 bbcode_ids defined in includes/bbcode.php, with any user-defined bbcodes being stored in the database using any bbcode_id >= 13. This means there will be a confilict if someone wishes to add a 13th bbcode to includes/bbcode.php and and user-defined bbcode already exists in the database (using bbcode_id=13).

I would suggest that NUM_CORE_BBCODES constant in includes/constants.php is increased in vlaue (ie. 20?), and and an sql script created to increase all bbcode_id values stored in the database to be bbcode_id>20