Yes, this has certainly, particularly with the big commercial mowers or trailing brush cutter units, but in 20 years years of doing this I’ve never broken even a window with a mower, so maybe the probability is low if you have a protocol and don’t get complacent. Mowing uneven ground is a big risk factor here. I can also confirm that golf balls have a higher probability of becoming projectiles than stones due to their elasticity: they are not sharp, but boy can they take flight with great velocity if your blade gets a hold of one. As an operator you need to consistently keep your eyes on the ground ahead of you and remain continually aware of where the clipping spray is going to go, because there can be rocks in the spray. An operator probably has a much greater likelihood of killing or blinding someone if they become complacent or lack situational awareness. Mower blades and fragments of mower blades can also go flying, but I’ve not encountered this in 20 years of lawn care. You’re more likely to damage the deck if a blade somehow comes loose.
In my experience the string trimmer, also called a weed whacker, is a far more dangerous piece of equipment where projectiles are concerned. It will often kick up small stones from the ground and hurl them in the wash direction. I have broken a few windows and would probably be blind by now from ricochet were it not for making it a practice to wear safety glasses. I don’t think that a rock hurled by a regular handheld trimmer could kill you, but maybe those high-powered two-handed brush cutter trimmers could do someone in if the wrong item got kicked up in the spray. Always pay attention to which direction you’re kicking stones and keep your safety glasses on. If it’s a hot day and the glasses start to fog up on you, put them a little further down your nose to keep the moist air from building up in there. That way at least they remain on, mostly protecting your eyes. Observe this with the motorized hedge clippers as well, they throw sticks and little projectiles at you quite a lot.