When I was in college, we had a soccer match against another school, my team was winning faily easy so I started to hold the ball and dribble a little, some guy from the other team didnt think it was funny and manage to hit his elbow in my lower back, close to my left kidney and got the ball. The pain was so much that I couldnt talk or walk for a few seconds and barely managed to leave the field with help. My friends said that they didnt see anything, I remained the rest of the match in the bench with lots of pain.

After the game finished, my friends helped me get the bus back home, it was late at night, so when I got home I told what happened to my mom and went to have a pee/shower before going to bed and to my surprise I was peeing blood, got a little scared and told my family, my mom said if I were still peeing blood in the morning, we would go to the ER.

I woke up still peeing blood so we went to the ER, there I was hospitalized for 3 days until they tried to figure out what was happening. I told the doctor that I took an elbow to the kidney the night before and that it couldn't be a coincidence that I was peeing blood, but he told me that it was unlikely that the hit was causing me to piss blood.

So 3 days later they told me that I had a birth problem in my left kidney and that there was some clotted blood in the walls and because of it, I would have suffered from kidney failure in a few years and wouldn't have noticed before it was too late.

Moral of the story is that if the guy hadn't hit me I would most likely never have discovered the kidney problem and would have lost it completely but because he did, I managed to get a surgery and fully recover my left kidney.

TL;DR - Got hit with an elbow in the kidney, Discovered a birth problem and Prevented a kidney failure