We had to muzzle one of ours for a while after a new vet tech tried to give the Kennel Cough vaccine (which they HATE!) and held the thing in front of his nose for quite some time while trying to align it. This made him scared and growl a little like they do. She also was a little scared by the sound he made and he sensed that.
IT was not made worse by the fact that in spite of my asking them not to, some of the techs would walk from the back door with the muzzle, holding it out for him to see. This made it worse as all he saw was something he did not like coming at him for a long ways away.
Eventually he would growl at anyone wearing scrubs.
It took us quite a while to get him back to being able to be treated without a muzzle by working with the vet directly.