I have encountered that as well, unfortunately. It has ben on rare occasions, but at our dog park some people think it is funny if their much smaller dog picks a fight with Gunther. I have alot of people to back me up as we go there on a very regular basis, and Gunther is very shy around new dogs, to the point of being fear aggressive towards them so I find this totally reckless on the part of the owners. Of course if it comes to the point where he finally snaps at them b/c they aren't taking his hints, well then he is a big mean dog.
Thankfully I have him trained to the point where I can see his breaking point and tell him to "leave it" and he will try his hardest to walk away from the situation, which isn't always easy with a dog getting in your face. I usually try to stay out of it if at all possible, b/c he's a big enough mama's boy the way it is, I prefer if he can work out his problems on his own. But I have had to jump in if it goes from just picking on him to full on aggression (which surprisingly we have not had an occurence of since he's been 1 yr old......for awhile there it was fairly frequent)