In 6.6.7 and 6.5.25 a bug was fixed "It was possible to delete a completed procedure from the appt edit window even if no permission."
This also happened in 6.4.22 you are also able to delete completed procedures from the appt edit window - though I'm not sure whether this was with permissions or not.
I wonder whether the delete button should be removed and we should not be able to delete procedures from the Appt edit window. Or at least, we should not be able to delete completed procedures from the Appt edit window.