1. Create a course with a student and an assignment
2. View all submissions for the assignment and check the "Quick grading" checkbox
3. Enter some feedback for the student and save
4. Navigate to the gradebook for the course and turn editing on
5. Edit the student's grade for the assignment, modifying the feedback
6. Save

In this case, the pre-existing feedback from the assignment trumps the edit (which is expected) however, it's not at all clear from the UI that this is what's happening. One solution could be to grey out the feedback text area unless the "Overridden" checkbox is checked.