I cannot believe they put her under again! How could they feel that was safe? I don't understand the original mistake, but then to top it off with that! (fuming) and then act like they were doing you a favor

and why was her head wet?
I also thought when you have your animal spayed/neutered, there was paperwork to be filled out consenting to that specific procedure - another oops.
What if your dog was never spayed and you didn't want her to be?
And I think one of the biggest reasons people in the medical field get sued is not because they made a mistake, but because they then don't own up to it and apologize. If somebody says we totally screwed up, we are so sorry, yes, I am still mad, but if nobody ever calls to even say were so sorry, then I am wanting them to know what they have done to me and how they have hurt me and wanting them to pay.
And as you said, you can see the scar. My girl was spayed over a year ago and I can see the scar - why can't a trained vet?
I'm sorry that happened. Hope she recovers quickly. Might consider another vet, I sure would.