Really? Have you ever actually heard of that happening?
Does that mean that when fishing a 3oz lead you'd need a shock leader in case the same happens? Or when casting a loaded feeder?
What about catapults. What will we to to prevent an overshoot which might hit someone?
Common sense in most cases. Obviously, a shortish range lob isn't going to be a problem but a full bloodied cast could be lethal if a crack off occurs. And to answer your question, yes, I have seen crack off's a number of times, mainly a long time ago when we were using lighter lines and leads were getting heavier, but I've seen idiots crack off on spods as well.
However, if you cause injury or damage through your actions, you are responsible legally. If you have been reckless, you may face criminal charges. If you are using mono, a shock leader as others have described will limit the possibility of this happening, or use heavier braid as Sam describes.
Law, I hope I'm not on the opposite bank when you're spodding. Body armour and a helmet aren't part of my angling clothing!
Stu