Yes, when I lived in the city 20 or more years ago. I had a very big fear aggressive boy, but had trained him to the point of ignoring everyone else in favour of me and it was by then extremely unusual, if not unheard of for him to respond to strangers at all.
We were walking towards the exit of our local park one evening (winter, so already dark) when a rather menacing chap stopped, squared up, blocked the exit and stood there staring at me/us, before starting to walk deliberately towards us in a very aggressive way. It was very obvious he was about to do something nasty.
We had already had some problems with people we felt were interesting in getting hold of our dog for dog-fighting purposes (notorious area and someone had even broken into our house to try and get to the dogs) and I think in retrospect his aim was probably to get the dog, but he hadn't reckoned on my lad being so protective of me. First he shifted so that he'd put himself between me and the attacker, then he reared up and went absolutely ballistic at him. The guy tried to stand his ground, then tried to bluff it and get closer, but didn't stand a chance and ended up panicking and running away. I have honestly never been so scared in my life, it was horrible and left me shaking like a leaf.
My first ever dog - enormous long-coated GSD bitch was very protective, but always in a quiet understated manner. When walking alone her at night or in lonely places I always felt that she was hyper-alert and that made me feel safe. She was a rescue we'd found almost dead under a hedge at about 7 years old and we discovered by accident, when a friend was messing around playing with her, that if you grabbed her collar and said 'attack' she did indeed rear up and bark furiously - presumably someone had had a go at something akin to schutzhund with her at some point. Needless to say we never did that again, but I did feel had I ever been in a dodgy situation she would have protected me.
Interestingly, my other dog was a GSD cross, so maybe it's the guarding side of their nature that made it more instinctive for them.
I have had 5 other dogs of varying breeds over the years and I don't think any of them would have protected me instinctively. My current two would probably run away and being Lurchers, would be out of sight in seconds. 