During the period leading up to my examinations, I had thought that my material had been prepared fairly well. Approximately two weeks before the first examination was due, I decided to do some hidden questions on how well-prepared I was.
I had four papers, and so I opened up four numbered doors (a random unknown number for each subject) in the image stream. (Okay, I admit...Alex gave me the idea for that and guided me through the process.) As it happened, two of the papers needed more attention, when I had thought that they were all right and needed only some extra effort. The result was that I radically changed my revision schedule leading up to the examination, and it paid off. As is to be expected, the image streams were right.