I taught at the aviation college of a mid-West US university in the late 90's. It became obvious that many students were cribbing during multiple-choice tests.. The next time I produced 3 sets of identical questions, but they were in a different order on each set, the set being discretely marked in one corner. Of course, many of the students failed miserably and there was much wailing and gnashing of teeth. I just shrugged and said that was the way they came out of the computer - which was entirely true. No-one had noticed the discrete set annotation.