OMG! War stories are the best!
For me, Three coops and runs built here without injury. (knock on wood because I have two left to build and they're big ones!) But heres my story from building my mounting block (that a step ladder or crate to make getting on your horse easy, for the non-horsey folks)
When I was building my mounting block I was nailing it together, and I was on the second to last nail, and I hit the nails really hard to make them go in with less strokes. Unfortulately, I hit my thumb and it HURT!!!! So my head is swimming, but I'm nearly done, and I tough it out until I got the last nail in, and I walk into the kitchen to get some ice, and grab my gel pak from the freezer and this point I'm about blacking out from the pain. In fact, I laid down (collapsed) on the kitchen floor util I had control of my faculties.
My thumbnail turned black, and a week or so later I went to the doc to help me out and they used (you won't beleive this) a paper clip heated up with a lighter to punch a hole in the nail to drain the bloodblister beneath it. It took two years to grow back looking normal.
I learned the hard way to use pliers to hold nails I need to hammer in. I stole that idea from this old house. And I wear work gloves when I'm doing anything at all outside, especially handling wood and hardcloth. I've had too many chunks of hide pulled out over the years.