My Father-In-Law has owned his been a building contractor for almost 60 years. In all those years of using power tools (saws, nail guns, etc.) and working in high places on scaffolds and ladders, he had never had any serious accidents until about 2 weeks ago. While nailing a cross-member the nail gun kicked back and then came back down. He still had his finger on the trigger and when it hit the second time, the nail gun fired and sent a nail right through his finger. It then rebounded once more, this time with his finger off the trigger, but came back down on his thumb, and the weight of the nail gun plus the fact he was leaning into the work caused it to shatter the end of his thumb. All of this happened within a couple of seconds.
Luckily, the nail missed the bone in his finger, so no major damage. The thumb actually hurt more than the finger did. A few weeks of soreness and all will be good.
I have attached a photo of his hand.
WARNING: Photo is VERY graphic. DO NOT click link if you are the least bit squeamish!! 
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