Steps to reproduce:
1. Quiz locked from 'Edit Categories & Items'
2. Tutor goes to review of student quiz attempt and amends the grade of one question using 'Make comment or override grade' (Grade becomes 0 from 1). The tutor saves the changes to the grade.
3. The resultant pop up window indicates that the grading item is locked (as expected), but contains the following error message:
Test Question MC - multiple answers (3)
Grading item is locked!
line 175 of lib/gradelib.php: call to debugging()
line 270 of mod/quiz/lib.php: call to grade_update()
line 478 of mod/quiz/locallib.php: call to quiz_update_grades()
line 69 of mod/quiz/comment.php: call to quiz_save_best_grade()