Never anything serious, but I have had a similar experience indeed.

When I was a baby they discovered I had a condition called Epididymitis (I won't explain it here. Look it up in Wikipedia. And yes, I was cured of it) and in the process of the diagnosis they had to X-ray me. Being a small baby, X-raying one part of me meant other parts could be seen in the scan also, and the doctors discovered something odd about my kidneys: that there was only one (on my left). I must have been born that way, it can happen. I think the condition is called unilateral renal agenesis or something.

So that's how I discovered I have only one kidney. I suppose it could be potentially risky had I never found out, as the discovery led to doctors' order not to play any contact sports (Rugby, American Football, Martial Arts etc). If I did play them, and happened to hurt my kidney in the process, I could well be screwed.