I came across a strange error with quizzes. I use a number of "Description" items to separate groups of questions. Moodle 2.1.1+ wants to count some of these as real questions. Of course, the student cannot get these correct, so the grade calculation is wrong. I think this has something to do with groupings of "Description" items. In the quiz. Notice in this graphic that I have 5 groups of "i's". Moodle thinks this quiz is worth 32 points, but it really only has 27 questions. The best that a student can score is 27/32.
I made a tracker item for this. (see MDL-29270)