It is possible to make the gradebook override an assignment grade without actually overriding it. Steps to reproduce:
1. Create an assignment (any type).
2. In the assignment submissions/grading screen, turn on 'Allow Quick Grading'.
3. Enter a comment directly into the quick feedback field of one of the students. During the comment, hit the Enter key at least once, so a carriage return is included in the comment.
4. Enter a comment for a different student, this time not hitting the Enter key during the comment.
5. Click to 'Save all my feedback'.
6. Click to 'See all course grades' (go to the grader report)
7. Take note that neither grade has been overridden yet.
8. Turn editing on, and WITHOUT changing anything, click Update.
9. Verify that the first grade entered is now overridden (the one with the carriage return), and the second grade has been left alone.
This is frustrating for our teaching staff as they routinely have to modify/revise grades, but they are 'locked' in the assignment grading screen due to the override. The only solutions are to A) Edit the grade directly in the grader report (we prefer not) or B) Untick the override option on the grade (fiddly, hard to explain, and doesn't always work as expected).
I'm assuming the presence of a carriage return is causing the 'old' (prior to turning editing on in gradebook) and 'new' (after turning editing on in gradebook) feedback values to appear different, thereby forcing the grade to be overridden during the save process.