The recurring appointments created at the first of the year, we cannot delete one of those appointments.
Steps to Duplicate.
1. Highlight the appointment and hit Delete key.
2. Choose to delete from All Mailboxes.
3. Message box asks if you want to delete This Instance, All Instances, etic.
4. Select This Instance button.
5. The system acts like it is accepting the request, but the appointment is not deleted from any mailboxes.
Please note: The process works for recently set up appointments, just not for appointments set at the first of the year.