The notification email will fail (partly) to send if there is a custom field (i.e. user or group picker) which has already been deleted from the system.

* Current Assignee  (Delete)
* Reporter (Delete)
* All Watchers (Delete)
* User Custom Field Value (Multi User pIcker) (Delete)
* Single Email Address (tchin@atlassian.com) (Delete)
* Single User (admin) (Delete)

If the notifications for an event is set up like the above and if "Multi User pIcker" is removed from JIRA, the notification email number 4 and 5 will fail.