There were some times as a kid when there would be shots I wasn't expecting, so that was always unpleasant, but the one time which stands out in my mind, I've written about on other threads before.
It was an employment physical, with a med student observing, and my doctor thought it was going to be pretty quick, just checking vitals, and filling out the form I brought. It turned out the med student knew someone close to me, so we were chatting.
Then, as the doctor was going down the form he realized they were asking for more. He said I could keep my shirt on, but take off my pants and underwear, and bend over the exam table for a prostate check. With the med student looking on, the doctor slid a lubed finger inside me and felt around. Then, he handed me a tissue to wipe the excess lube.
Afterwards, the doctor went back to filling out the form, while I stood there with my hands on my hips, naked from the waist down, for a minute or so, not sure what was next. The med student noticed me just standing there and got the doctor's attention. He said I could get dressed again.
I remember feeling rather shaken by the experience. One moment I'm casually chatting with the doctor and med student, and the next I'm undressed and being probed internally in front of them, and then left standing there, still exposed. I think what bothered me more than anything was wiping in front of these two afterwards. A little difficult to remain so casual as before, after that.
Another experience from a medical situation which, while disturbing at the time, has been endlessly arousing to think back on ever since.