I was just creating a new quiz and it shared some questions with another quiz that students have already taken. In order, to make sure I was creating the quiz the way I wanted it, I had the new and old quizzes open in separate tabs in Firefox. I made some changes to the new quiz, and then clicked save changes, only to have the changes affect the old quiz! The number of questions shown in the old quiz had been about 88 but it was reduced to the number in the new quiz, 15, consisting of the first 15 of the 88, and the grades disappeared. I became very confused at this point and don't know what I actually did next but suddenly the students' totals on the old quiz became something like 1137%. I think I can recover everything manually in the database, because all that seems to have changed was the list of questions in the quiz and the student grade totals, but this obviously could wreak havoc if someone were to reproduce it.