Gotta love the four foots....always have.
There are two things I know of which must be remembered....in addition to how a dog is trained.
1) Inbreeding. Unscrupulous breeders can produce "wacky", unpredictable animals.
EX: Cocker Spaniels in the '20's, German Shepherds in the '50's (think Rin Tin Tin), Dalmatians (think 101 of them), Pit Bulls (think drug dealers in the '80's protecting their turf).
2) Confined and isolated dogs. Many people don't exercise their dogs and the more "alpha inclined" ones can become explosive. While regrettable, this can happen innocently.