I've seen something along these lines which is a law suit waiting to happen. A teaching pro that operates at our club has kids run along the court perimiter while hitting balls at them, often at their legs and feet causing them to jib and dodge the balls. Of course they think it's hilarious. Having seen the results of a friend step on a ball I think it's stupid.
Last week a family was taking lesson with this pro. He had mom, dad, and the three kids running the same "dril". Sure enough, a boy steps on a ball while running and goes down. He lays on the ground crying for a moment, the pro tells him it's no big deal, and he gets up eventually. I couldn't help but think how lucky the pro and the kid were since the kid didn't face plant and break his nose, crack his skull, or break an arm.
Another time my wife (!) entered the court behind me and without my knowledge. I was warming up and nearly clobbered her with a back swing. She readily admitted it was her fault but i wonder what would have happened had it been a stranger and I actually hit her.