I genuinely have never witnessed it myself, I have 2 dogs of my own, however there have been a few incidents local to me that i know about.
Once with 3 large off lead dogs jumping up at my 3 yo nephew, the owner had no control and actually said to my mum who was with my nephew 'well what do you expect me to do,I can't control them'!
There have been a couple of worse incidents, all by off lead dogs, 3 x staffs, 1 Akita, plus a pack of huskies. All were attacks on dogs, varying severity from minor injuries to the pack of huskies killing a puppy in a dog park. None on humans.
I really don't think it happens often but is horrific when it does, I do find it very frustrating when owners have no controlbut most I have met generally do, all dogs have the odd lapse. we keep ours on leads unless in a large green space, newpup only on a long line till we have trained full recall. My eldest dog is very barky, he has quite bad anxiety, we work really hard with him on it but I'm sure people think we just let him bark. He's really well behaved in the park and off lead, it's mainly on lead or in places where people walk close to his face, eg a cafe.
Trouble is you don't actually know when a dog is really friendly and just a bit untrained or goofy or whether they could be dangerous. I think there are more reports of out of control dogs than actually really happen due to lack of understanding but there are some really shit owners out there who ruin it for the majority.