As someone who worked as a vet tech for a long time, believe me when I say that nothing your dog could do would be all that surprising. I've been peed on, pooped on, had my leg mounted...and every surface on the clinic was probably marked or pooped on/near at some point.
When they say it's not the first time and won't be the last, they mean it.
My funniest, working there:
Once I was sitting on the edge of the exam table holding a puppy up in a human sitting position so the vet could look at his tummy, and the pup peed on me. Since my lap and hers were lined up, it totally looked like I peed myself.
I used to bring my son to work. It was a small family-oriented practice. My son was potty trained enough that he could go by himself, and in the middle of a waiting room full of clients, he came out of the restroom with the pants and underwear around his ankles and asked me to wipe his butt.