Maybe not the dumbest thing I've ever done, but close, and it did hurt when I landed. I engaged in a parallel tree climbing contest with some siblings and friends at the back of our yard, where there was a hedgerow of poplar trees.
The trees were not those spreading cottonwoods like you see along rivers. They were the tall skinny kind. Those have, as I discovered, increasingly thin and brittle branches as one nears the top, plus it's harder and harder to get a foothold as the spaces between limb and trunk become more narrow.
My sibs and pals drew benefit from my determination to prevail in the contest, since as I climbed past them, toiling away for their next hand and footholds in adjacent trees, they witnessed what happened when I "won" but the tree promptly ejected me for stepping on a branch that had tolerated a step up onto it, but wouldn't take my full weight as I went for the next handhold, so I was in for a fall of maybe 15 or 20 feet.
I'm lucky I only got a couple hairline fractures of some fingers, probably because I was still hanging onto the branch that broke in my departure from the tree and came along for the ride... I used it to try to break my fall and save my neck. Anyway my full weight went onto my hands around that branch as I tried to handstand and somersault my way out of a head-on collision with the ground. I was about 11 years old, so being relatively lightweight probably saved my life.
Ugh. I had to endure jokes at the hospital from an intern as he looked at the Xrays: "Wow, hope you don't play the piano" -- which, of course, I did. The rest of my payback was hearing "You should have known better" from parents, and a jeering chorus of "Yeah!" from the sibs. I've been tempted to climb a few trees since then but managed to resist, so I guess I learned something.
Ooooof.
That sounds as though it was a painful experience.
In a way, this tale reminds me of a friend, who, a few years ago, thought - casually - to prune a large tree (an oak, I think) in his back garden.
A sturdy branch he was on - I never did find out whether standing on or sitting on the thing - sprang back - in fact, catapulted back - hurling him skywards and then gravity kicked in leading to a collision with the ground. Both his legs were broken as a result.