My Pit mix was great at the vets, except one time. This was back about 15 years ago, and the vet was giving Gus one of the vaccines that was dripped into the nose. He had just had an unpleasant time with the Vet expressing an infected Anal gland and it must have hurt a lot, as Gus grunted several times as it was done.
I had Gus in a headlock and he growled as the vet started to drip it. "Hold him" he says, like it's an easy thing to do! He had to stop the drip and then he started again, and Gus went "RRRRRRRRRRRRRRRRRRRRRRRRRRRR!" loudly. Something he almost never did. Finally, as the vet finished, he was pretty panicky, and growled a third time. The vet took a step back, and I let go, and BLAM, he attempted to remove the vet's face! He lunged at him and actually jumped off the table, snapped his jaw shut like it was some sort of machine, and ended up on the floor. The vet had just started to back up as Gus made his move, and that's what saved him. He shook his head and said over and over, "Whew!! I could feel his lips and teeth on my face! Wow!!" The receptionist (his girlfriend) came into the room, and saw how shook up he was, and says, "What's the matter?" He says "Gus tried to take me out!" She couldn't beleive it, "Gus??" He says, "I pushed him too far, and he went for me, it's my fault, I ignored the growls". That was Gus's one and only actual attack, ever. The vet and he got along fine after that, but the vet's son told me there were several dogs his dad really "respected", and Gus was one of them.