My own dog, Fozzie, the Golden mix which I adopted at 8 weeks old from the pound, has "turned" twice. This was really frightening and disheartening to me as he is such a goofy lovable dog...to everyone. One time was when my friend and I were in my pool. Fozzie was on the deck looking down at my friend who was talking to him, and Fozzie became aggressive and barking threateningly at her. Not just once or twice, but extendedly. It scared her (and me), and I still have no explanation. The other time was when my twin grandsons invited a little boy to the pool. The three of them were standing on the deck before entering the pool, and Fozzie began barking aggressively at the little boy, who just stood there frozen. I scolded and grabbed Fozzie and took him inside. I asked the little boy what happened, and he said he just looked Fozzie "in the eyes." Both of these incidents had to do with my pool, and I've noticed how Fozzie doesn't want my other dog on the pool deck. He seems to think it's HIS territory. I'm not sure what to do about that, but I can't have him threatening my visitors. He has never bitten anyone, but he can be quite rambunctious and hard to control. On walks, after he knocked me down trying to get away from me as other walkers approached with a dog on a leash, I did some research and bought a "head halter." This has certainly helped. It fits over the nose and up behind the ears, and makes him behave when we walk. He doesn't like it, but it's painless, and since he loves going for a walk, he lets me put it on him.
Sorry for rambling. I do say with certainty, however, if Fozzie ever bit someone, I'd have to put him down. He's big and hard for me to control.