There are two parts to this issue:

1) 40 students in two groups in one course did a 21-item quiz (7 questions over 3 pages). One student noticed that a short answer question that should have been correct was graded as wrong, so I edited the question.

2) This necessitated me to regrade all attempts which I did so by selecting all attempts in the Results page and clicking on the [Regrade selected attempts] button. The page reloads with the following error:

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No attempts need regrading

More information about this error

line 429 of /lib/setuplib.php: moodle_exception thrown
line 670 of /mod/quiz/report/overview/report.php: call to print_error()
line 140 of /mod/quiz/report/overview/report.php: call to quiz_overview_report->regrade_selected_attempts()
line 90 of /mod/quiz/report.php: call to quiz_overview_report->display()

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So no attempts were regraded, but students who had written the correct answer for the edited question can see in the review page of the quiz that it is now correct, but in the 'navigation block' that appears to the top right of a quiz that same question is marked as incorrect. I have attached two screen shots to illustrate this. (Also, I noticed that while students were doing the quiz I noticed that questions in this 'navigation block' were being marked as unanswered, whereas in fact they had been answered.)

Finally this issue is related to MDL-27301

Interestingly, it was essentially the same quiz, with the important difference that in the quiz reported in MDL-27301, the 21 questions are in the question bank for that course, BUT in this case the 21 questions are in the System category. The other difference is that in this issue the 40 students are divided into two groups. In MDL-27301 the students are in one cohort.