Email notification on issue Note update/modification
There are permissions allowing to edit (own) notes but after such modification no email notification is send.
It seems a usable feature to modify notes, correct mistakes or maybe specify more info in single place. Unfortunately, email notification is send only when the note is created and further modifications may stay unnoticed by watchers.
It would be good to see another option "Note modified" under Settings->Email Notifications->Select actions for which email notifications should be sent->Issue updated.