1. Using Resource Assignments, create an assignment for a unit other than your own and make it repeating.
2. Select one of the repeating events on the assignments calendar view, select to edit "Only this event".
3. Make a modification to this one event and save it to create the exception.
4. Select to edit this exception again, and I receive the message:
Requested event belongs to a unit that is not related to user
I can edit the series that was assigned to the other unit, but I can't edit this one exception.