I haven't gotten to the point where I throw curve balls during a test yet, but likely will as students progress. My own instructor had a policy for black belt tests that something had to be corrected on the spot. He'd pick something that was arguably wrong, and make you correct it before you could go to the next technique. The point was to put you under some stress, and to see if you could make a correction on the spot (something he feels is important for that rank/level).
I've thought about the idea of testing in front of a group. We never did it. Most tests were just the testee, the instructor (maybe an instructor-in-training, too), and a partner for the parts that required a partner. The scenario tests included all the students of a specified rank, but the doors were closed to everyone else - both students and observers. I like the idea of putting at least some of that in front of a group, though that only works when a group is available. Do you often have non-student observers for adult tests?