Every year it happens to someone in my area. 5 or 6 years ago a close friend who does use a harness fell. He had not yet attached to the tree and was about to step on the stand and his foot slipped off the top foot peg. He was falling backward but momentarily grabbed the stand on his way down, This up righted him and he fell 22 feet and landed feet first. Broke both bones below the knee on one legg and broke the fibula in two places on the other. He now has steel rods from his knees to his ankles in both leggs. After a long hospital stay and rehab, he had to learn to walk all over again.
Then a couple weeks ago another friend stepped on his stand and before he fastened his harness to the tree, the ratchet strap on the stand broke and he fell 18 feet. He suffered 3 fractured vertebrae, a fractured sternum, and a broken ankle. He is home from the hospital but Doctors are waiting on the swelling to go down in the ankle so they can do surgery on it.
A few years ago a bolt failed in my seat climber as I started down one morning. I fell backward but thankfully my harness caught me and that was it. Guys, it is not IF you are going to fall, it is just WHEN!