Working at one place some years ago, they had a scaffolding tower with inbuilt ladder up. Everybody told under no circumstances were they to go anywhere near it. It was fenced off.
Huge (6 foot 3), incredibly athletic, parkour loving 16 year old climbed over the barrier and decided to go to the top. All well and good, he could easily support his own weight one handed. He decided to come down the outside of the tower, having shown that he knew 'exactly' what he was doing at height ignoring the fact that he obviously didn't know that inbuilt ladders are designed to be used from inside the tower.
Got to the bottom rung and caught his foot. He pitched forward onto his face, his foot stayed where it was as he fell an entire two foot. The resulting damage meant that he practically severed his foot internally and will not be ever be doing Parkour (or climbing, dancing, or walking without a limp or pain) again. Had he done exactly the same further up the tower, it would have been catastrophic, rather than just permanently disabling.
He might have got hurt any other time, but it was the moment he was above ground level for more than jumping (and obviously, not wearing the sort of boots that builders/site team are required to wear) that he caused himself permanent damage.
Guess he learned about risk, though.