When I was still a teenager, maybe around 15 or so, I developed some dark brown spots on my abdomen. They were not like a rash, or insect bites, maybe more like a discoloring of the skin in a somewhat oval-like but random pattern. And they kept getting larger and larger.
I already knew then (from reading in my grandfather's old self-help medical book) that skin discoloring is often a sign or precursor to many types of skin cancer. So, somewhat fearful, I nevertheless told my parents (but nothing about my thinking it skin cancer) and they made a doctor's appointment.
Well, it turned out to be very little, just some kind of rash caused by something that I now forget. The doctor quickly examined me, chuckled a bit and prescribed some sort of peroxide solution which ended up doing the trick and the discoloring went away.
But before I left, he asked me with a rather silly grin on his face 'what I thought it had been? why was I so worried?' And he pointed to my abdomen and made some indeterminate motions.
For some reason I didn't want him to think me a doomsday prophet, so I said nothing about my thoughts on skin cancer. I just mumbled something about it being unusual and out of the ordinary and mumbled some other nonsense.
Later, I understood that he had been asking in a roundabout way if I thought the discoloring had come from masturbating. Since the spots were just on the region I would have ejaculated on when lying on my back while jerking off. It took me a while to figure out what he had been asking. And then I was sorry I hadn't shown off my very rudimentary medical knowledge. He'd thought I was just another horny teenager with no knowledge whatsoever.