Years ago we had a big, dumb, friendly female german shepherd mix that would check out other dogs that might be walking down our street, but never seemed to show interest in people walking down the street.
One day I was working in the garden and the dog was hanging around with me when suddenly she took off running, giving a couple of short, mean sounding barks. I turned and saw she was heading right at a jogger. I shouted out NO! and took off after her. She stopped about 10 ft from the jogger, who was frightened out of wits. I apologized up & down to the guy, and felt like a heel.
I really never thought she would chase someone.