When using the "Mark all as read" link in your inbox, and an error occurs (e.g. if your Internet connection is spotty), regardless of the number of unread notifications, you'll get this error message:
An error occurred while updating the status of the notification. Please try again.
which isn't pluralized. (I do appreciate just getting a single message though :))
An error occurred while updating your notification statuses. Please try again.
sound? Any other ideas on copy?