I try to be professional at all times during exams, but this one from a couple of years ago still stands out in my mind.
I was assisting during an annual in a clinic. The patient was a young attractive woman with big implants. When the Doc did the breast exam, perhaps it was just my particular view, but I noticed that the patient's unusually large and gravity defying breasts were individually much larger and rounder than the Docs head. This struck me as funny and I started to giggle (silently!) and had to leave the room suddenly and go have a laugh. The Doc chased me down around the corner- she thought I was sick or something-and let me cool off a little and we completed the exam. For some reason I still get a smile out of that.
I've also had two(!) experiences of males in the exam room fainting during the female's exam. One suffered a head injury and was hospitalized, the other we caught. This was early in my career and I was unprepared. Now I always keep one eye on any extraneous people in the room (I'm in the hospital now). My own DH almost fainted during the birth of our first child, but the sharp eyed nurse caught him before he hit the floor.
Finally, I actually fainted following a blood draw - my own. In all fairness, I was sick. I remember having the blood drawn, then moving to another chair while someone else was having their blood drawn. The next thing I know, there were all these faces in my face. I said "I'm sorry, I must have fallen asleep." They said "No, you did more than that." The moral of this story is, I guess, 'Your blood is fine with me. My blood, however, is a different story.'