I had an embarrassing one. I was 19 and having my Army draft physical. There were a bunch of guys, probably 50 or more, and we were all stripped to our undershorts (in those days mostly "tighty-whities").
After the various eye tests, hearing checks, etc., we got to the real part of the exam that the doctors had to do. We were herded as a group into a big room where we lined up around the perimeter while two doctors went around the room, from person to person, doing the heart/lung checks and, of course, the obligatory genital exam.
The last thing I thought I'd feel, with the prospects of getting drafted hanging over my head, was arousal. But as I looked around the room at the guys in their underwear, I spied one dude wearing a jockstrap, and I love those! As I watched the doctor examine him, pulling his jock down and fondling his balls, I popped wood in my briefs. And it wasn't too long before one of the doctors was in front of me, commanding my underwear be dropped.
Well, the now-partial boner popped out, I was examined, nothing was said, and I got drafted 3 months later. End of story.