Cross Joins are done using an approach like this:
table_a, table_b
but this generates an errore when there is also an inner join. This query, for example, is generated when trying to send a Mass Email to a group:

SELECT u.user_email, u.username, u.username_clean, u.user_lang, u.user_jabber, u.user_notify_type FROM phpbb_users u, phpbb_user_group ug LEFT JOIN phpbb_banlist b ON (u.user_id = b.ban_userid) WHERE ug.group_id = 8 AND ug.user_pending = 0 AND u.user_id = ug.user_id AND u.user_allow_massemail = 1 AND u.user_type IN (0, 3) AND (b.ban_id IS NULL OR b.ban_exclude = 1) ORDER BY u.user_lang, u.user_notify_type

and produces the following error:

SQLSTATE: 42000 code: 4104 message: [Microsoft][SQL Server Native Client 10.0][SQL Server]The multi-part identifier "u.user_id" could not be bound. [4104]

The problem can be solved using the "CROSS JOIN" keyword, instead of the comma, at row 612 of dbal.php file:

$sql .= $this->_sql_custom_build('FROM', implode(' CROSS JOIN ', $table_array));

After this change the query will be the following

SELECT u.user_email, u.username, u.username_clean, u.user_lang, u.user_jabber, u.user_notify_type FROM phpbb_users u CROSS JOIN phpbb_user_group ug LEFT JOIN phpbb_banlist b ON (u.user_id = b.ban_userid) WHERE ug.group_id = 8 AND ug.user_pending = 0 AND u.user_id = ug.user_id AND u.user_allow_massemail = 1 AND u.user_type IN (0, 3) AND (b.ban_id IS NULL OR b.ban_exclude = 1) ORDER BY u.user_lang, u.user_notify_type

and it will be correctly executed.