I haven't had anything fly at me yet, but I have sent a couple of stones flying.
Once when filtering down a hill, I heard a loud bang as I went passed a car. I had felt the back wheel go over a stone, but didn't realise a stone could hit that hard.
Another time a stone flew, across the pavement and hit a large sign about 6 feet off the ground, again with a very loud bang. I was lucky as this road goes past a station and there are quite a lot of peds. The stone would have been travelling at head height.
I've since been a lot more careful when I see debris on the road.