Friend who's a professional carpenter and roofer nailed his hand to the roof one day. The guy's a safety nut, and never makes mistakes like that. But then he did. Power equipment plus advancing age will get us.
		
		
	 
OK  I can top this!  ;-)
When adding an addition to our house that included a cold roof that first required a deck under the metal roofing, my wife offered to help up on the roof.  Now the roof slope is pretty mild, and I was thrilled with the SPUNK shown by my rather timid better half (might be 3/4)  .  I held the ladder and steadied her steps as she got on the roof deck.  We were putting down 1 inch boards onto 2x4 sleepers.  She had hammer in hand, and nails in a bucket. With brief instructions given her on the nailing schedule,  was pounding away about 10 feet down slope (fall protection).
Well,  all the sudden my wife let out a mournful yelp and clutched one hand in the other,  dropping her hammer.  I lifted up, and took the three strides to her side where I saw drops of blood dripping from her clutched hands.  "I'm a trained med first responder, so she trusted me when I asked to assess the injury.  Somehow,  And I can't imagine how,  she had driven a 10 penny nail through the web of skin between thumb and palm.  OMG!  Needless to say,  she does not "do blood" and went vasolvagle  on me, and I had to shoulder her down the ladder and off the roof.  Good thing there is not so much heft to her.  
That was all 30 years ago,  I remember,  but NEVER bring it up.
I still don't know how a person can drive a nail through the hand that is supposed to be holding the nail.  My guess is she "flinched" let go of the nail, and when the hammer fell a glancing blow and the nail bounced with enough energy to penetrate the nearby flesh.  
Be careful,  danger lurks!