Forums allows you to assign grade to users who are not "students" in the gradebook

Create a topic that is linked to a gradebook item.
Post a message as a non-student role (ie instructor or teaching assistant).
You are able to submit a grade for this forum posting.
However, the gradebook tool itself only allows grading of "student" roles and there is no way to edit this grade outside of forums. The Gradebook tool will not display this grade.

A symptom of this issue appeared in Gradebook2 (the original description for this jira):
When a graded forum post created by an instructor or admin is deleted, the grade is still associated with gradebook2. If the instructor tries to change the grade type when the gradebook is organized without categories, an error message is displayed about removing all entered scores before making the change. Since instructor and admin posts do not show up in the gradebook their scores can not be removed.