Here's a report of something that just happened on our server.
There was a period of high usage Monday night, as indicated by the server's CPU and memory monitoring service. A student was taking version 2 of a quiz. (This was a quiz that allowed up to 2 versions.) It was well within the global due time and the version's due time. The quiz only allowed one graded submission.
The student reported that she had all 12 answers correct (including green bars, confirmed with a screen shot). But for 4 of the problems, there was the report of "answers only checked, answers not recorded" and her scores for these problems were recorded as 0.
The answer log indicates that there were two submissions from this student. Both at the same time (at least to the minute). And all 12 submitted answers are the same each time. The cap of 1 submission should not have let a second submission happen.
The timing log for the course only indicates one submission from this student.
My theory is that the surge in traffic at this time led to two submissions. (Maybe she even clicked twice.) And the problems (all 24) were processed not necessarily in a rational order. For some, they were fully processed from the first submission and the student got credit. For others, there was some crossed wires where the second submission tried to record a score earlier than the first submission, but there was still recognition of an earlier submission so the score was recorded as 0. And when the first submission finally completed it did not overwrite the 0 score that was already written.
That's my current working theory anyway. Does that sound plausible to those who know better?