I've got a recent one as an adult.
As a kid, I had a pretty high free standing jump. I used to jump on everything just because I could. As an adult now, I decided to start practicing box jumps at the gym to see if I could work back up.
I work in a newly remodeled office and our kitchen counter/bar top is a poured concrete slab. I was the only one in the office and had just poured my first cup of coffee and for some reason thought to myself, "I bet I can jump up there." The counter is a standard height in America, 36 inches, far lower than I can jump in the gym. No problem.
I took my stance, squatted down and jumped. I was wearing jeans and some Chukka boots, probably sub-optimal jumping attire. The soles of my feet ended up landing on the corner of the polished concrete top edge and immediately slipped back towards the floor. My forward momentum caused my shins to come down on the corner with full force.
Ahhhhh that hurt like shit and is definitely going to bruise badly. Walked balk to my desk, defeated, and sat down. Then I felt something running down my leg. Pulled my jeans up and saw that my leg was covered in blood. The force had torn a hole through my skin and muscle and I could see my bone inside when I moved the skin around.
Ended up with 4 stitches, a nasty scar, and a humbling story.