I underwent early puberty and as a result had to have regular (2 monthly) medical exams by my GP to monitor my development. These exams always ended up in me being naked and sporting a full erection; which often developed much earlier in the exam; to my complete embarrassment. Due to being young, and not having been given any proper explanation, I did not really understand what was happening and certainly didn't associate these reactions as being sexual.
When I was about 19, I got knocked off my bike. I was taken to hospital in extreme pain and with increasing loss of sensation in my legs & pelvis. Apparently (I don't really remember everything due to the pain relief), the doctors were worried about a pelvis injury as I had signs of blooding from my urethra and over time was developing a strong erection. I'm not sure about this, but was later told this can be a sign of spinal / pelvic injury - can any of our medically-trained friend confirm this? To examine me properly they had to cut my clothes off & log rolled me to do a spinal check & DRE (again not completely sure why?). By this point mum had arrived at the hospital to see me being take off for scans, with a thin sheet over me, but not hiding the "tent". Fortunately, it turned out to be severe bruising and no spinal / pelvic injury. I'm sure this wasn't sexual arousal, but I've never really understood what it really was; but on being told / confirmed what happened it was again very embarrassing.