This just goes to show, there is no such thing as a totally 100% safe dog. Every single dog that's bitten someone started out never having bitten before...until it did. They all have the potential to cause harm under the right circumstance. The vast majority of the time, things are fine, and people tell themselves they understand their dogs or dogs in general and "I just know he's friendly". But the truth of the matter is, dogs are not humans. We cannot adequately communicate with them or any other animal. Any perceived "certainties" we have about dog behavior are just illusions. No matter how much you love your dog, no matter how much time you have spent around dogs, and no matter how much you think you know about them, you can neither read their minds, or tell the future. Anything is possible.