function permission_set in phpbb\db\migration\tool can grant regular users administrative/moderative permissions when applying permissions set to group via migrations.
Scenario:

array('permission.permission_set', array('GLOBAL_MODERATORS', 'm_thanks', 'group', true)),

Th problem is that GLOBAL_MODERATORS group has 2 roles assigned: ROLE_USER_FULL and ROLE_MOD_FULL. Due to this code the function selects the both assigned roles (including ROLE_USER_FULL) regardless of the role type and assigns them the m_ permission.
This ends up in granting some of the regular users extra moderative permissions. In theory, assigning a_ permission set to the group will lead to granting regular users admin permissions silently.