I've seen two people hit hard with balls, and I hit my wife once:
1) At a course in Jax., a friend of mine was teeing off on #1. The #9 green was very close to the teeing ground. My buddy hit a low slice that caught a dude in the neck and dropped him. My friend was so shaken up that he didn't want to finish the round.
2) Playing with a father and son-in-law in Memphis, the SIL had dribble a tee shot. The FIL drove to his ball about 125 yards away. The young guy hit a low bullet with a 3 wood and caught the older guy in the ribs. He hit the deck as well.
3) When my wife was learning to play, we were on a par 3 course. I was in a bunker and she was across the green. I said, "Head's up!', to which she replied, "I've got it". Well, I hit a terrible, low shot that caught her in the shin. She stays the hell out of the way now.