I take care of a big dog, a rescue, who has shown some aggressive tendencies. He's an adult, he's smart, he's well trained: he sits whenever I say "sit," lays down when I say "down," leaves it (small dog, stick, chicken bone in the alley) when I say leave it. He is never off-leash when strangers are near, he's walked with a harness which allows me to control him and move him to safety when he does pull. When we see a threat or temptation, we cross the street or take a different path through the park, I give him a task (like "sit, down, look at me, heel") so he's involved in something else instead of being distracted by other-little-dog-out-of-control. Our most recent confrontation - which took no more than 30 seconds beginning to departure- was when an 8-ish year old child about 12 feet away was looking at him, all of us stationary and me being very vigilant, and I told the little girl "stay away from this dog" (just to be cautious - she did not appear to know dog-safety rules) and I called to her mother 10 more feet away and on her cell phone paying no attention,, but she did not hear me, so I told the girl again to stay away. Just as I turned to get away from the child, the child RAN straight towards the dog's face and I pulled the dog away and out of the area and - thank you very much, saved that little girls life, since my dog would certainly have perceived this as a threat.
SO, while we hope every dog owner - especially with a rescue - is doing their best to train, control and take care of their dog, there are still a lot of variables related to HUMAN error that causes the human to be bitten by someone they'll never understand.
No such thing as a bad dog, but there may be people out there who just do the wrong thing.