I fainted once back when I was a student shadowing a physician. It turned out to be due to a previously undiagnosed medical condition that got diagnosed pretty quickly (if you're going to pass out, a doctor's office is a good place to do it), but it coincidentally happened while I was watching them suture up a guy's ear after he had a factory accident.
The first thing everyone assumed (including me), of course, was that I passed out from watching the procedure. But, unlike most people who come to and then feel somewhat embarrassed over it, my first reaction was worrying the poor patient would be worrying his ear was in worse condition than it was (it was just a little tear in the earlobe, nothing bad at all, and it just needed a couple sutures, but he hadn't yet actually seen it, just all the blood from it), and was quickly trying to reassure him that it wasn't that bad. And, just about the time I was starting to realize my embarrassment as well, the physicians started to joke around and compliment me for falling AWAY from the surgical field instead of into it, so that diffused it all.
It was only after reminding them that I had seen someone in much worse condition getting stitches the day before that we reasoned it wasn't just passing out from watching the procedure and I had a full physical done that revealed the medical condition.
I'm just disappointed now that this is on my mind, and the med students have recently dissected the vagus nerve in gross anatomy and should know its function that I wasn't in the room where one of our students passed out this week. I only found out afterward since I was working in the room across the hall (we have two labs for gross anatomy). They told me he was pretty embarrassed over it, especially when they wouldn't let him get right back up...I would have told him he wasn't allowed to get up until he told me which nerve was responsible for the reaction he had, and what systems were involved.

(That would have bought enough time distracting him until he was ready to sit up.) I suspect in his case, it was likely extreme fatigue or onset of flu symptoms (grr...the med students are passing around flu bugs! I'm tempted to wear a surgical mask to class to keep them from infecting me, and am hoping that the aching muscles I have today really are just aching muscles).