== Invalidity case 1 == misc/cronjobs/overdue_notices.pl just silently does nothing when "Restrict" ticked and nothing else in the same trigger. A notification must be also enabled for the restriction to happen. == Invalidity case 2 == If "Letter" is set to "No notice" , then when "Email" is ticked, it's also silently ignored. The user should know about the trigger invalidity in the above two case to cause less guesswork when restriction aren't done or notifications not sent. Could be in the UI, could be the cron job that errors.
I think this is somewhat similar to the issues reported on bug 25049 - more validation is needed.