A friend of mine had a dog from a rescue centre (a German shepherd cross - can't remember what the cross was) Lovely dog, very friendly and appeared good natured. They used a dog trainer to help assess the dog and all seemed to going well. Then after there months or so, one evening the dog started growling. Next day all was fine. That night, the same thing happened. This continued for a few days, then, in the evenings, the dog started barring its teeth. During the day, all was fine. This carried on for two days, with increasing concern to my friend, who contacted the dog trainer. The dog was taken back to the rescue centre that day.
The main problem was that there was no known history about the dog, but it appeared entirely docile for about three months. There was nothing obvious that triggered the aggressive behaviour, it was unpredicted and unpredictable.