Here's the scenario:
1/ a student attempts a quiz
2/ the student's account is deleted from the system by the admin
3/ the teacher wants to modify the quiz (add or delete questions)
4/ in order to "unlock" the quiz, the teacher deletes all attempts that have been made (including the attempt made by the deleted student)
5/ no attempts are shown in the quiz report, but the quiz remains "locked" and thus cannot be modified; in fact, even though no attempts are shown, on the quiz overview page there is still info saying that there is an attempt

As a result, the teacher is unable to unlock the quiz and modify it (add or remove questions).

The workaround is to enter the database, "un-delete" the student's account, re-enrol him in the course, delete his attempt and then delete the account again. However, this requires admin intervention and tampering with the database