I had something similar happen to me...in my driveway. I had just gotten sliders put on my old truck and was trying the hi-lift out on them. I figured I'd get a hang for jacking up the truck on the sliders in a flat controlled environment like a driveway.
Anyways, I jacked it up and all was good. Flipped the lever to start jacking down and started pumping. First pin step down was fine but then it slipped and moved two pin holes down while flipping the handle up. The open end of the handle caught me in the side of the neck right below the jaw.
It hurt like a mother and knocked the wind out of me. I touched my neck where it hit and pulled away with a bloody hand. I couldn't see the damage but assumed I had a big hole and that I was in trouble. I ran inside, applied pressure then took a look.
Just a small hole, more of a tear than a cut. I got really lucky and learned a bit. To this day after a few days of not shaving I can see where the handle hot as hair doesn't grow there anymore. For the most part the hi lift lives in the garage, even when I leave on trips.