I've made plenty of mistakes in the last 65years or so.
The sad thing is that I continue to make mistakes almost daily.
I screwed up royally yesterday and feel terrible.
I hatch anywhere between 50 and 200 chicks a year. I used to leg band with numbered colored bandettes. It worked pretty well except when the birds lost their bands.
If enough lost them before I knew it, I wasn't positive who was who.
I switched to numbered wing bands a couple years ago because - even though I can't read them from afar- they never lose them throughout life.
I was OK with that till yesterday. I wing banded really young chicks (less than a week old). I will never wing band chicks under 3 weeks old again.
The band is supposed to go through the wing web between the tendon and midway between the wing bones.
In this case the wing was so small, it was hard to find the point to penetrate. The last one I did yesterday I really screwed up. It went right through the joint between the ulna and humerus.

Essentially I blew out its wing bones. All that was left was a little string of skin. After my shock, I decided to amputate. There wasn't much blood and it didn't complain much.
It is doing fine today except short one wing.