I am so sorry for your loss. Vets are only human just like the rest of us. And as such, they make mistakes. The most horrific animal death I have ever witnessed was a botched castration. Perfectly routine, textbook procedure. Vet was fresh out of school, came highly recommended by the regular vet who couldn't do the procedure that day. He missed an enlarged inguinal ring and the animal was eviscerated. He didn't believe me when I called to say the poor creature's guts were out. I waited an hour for him to come back. Another hour for the vet to clean the intestines and stuff them back in. He ended up dying in my arms... with only two stitches to go. I will never, ever forget walking into the barn that day and seeing him stagger to his feet and trying to come to me with his guts hanging out. It still haunts me in my dreams and was nearly two years ago... Do I blame the vet? Not really. Yes, it was entirely preventable, but we all make mistakes. I bet you that vet will NEVER make that mistake again. It is a shame that an innocent creature I loved had to suffer so horrifically, but his ultimate sacrifice made the world better for other creatures.........
My old dog had a major issue with his ear - an autoimmune thing that made it extremely painful to open his mouth. Our vet had NO idea what it was, and we spent SO much money trying to figure out what he had. For months we tried, until he figured it out - but it was too late, and he had to be put to sleep. But not long after that, another Mastiff came into the vet with the same issue, and he knew exactly how to treat it. It was awful, and we all wish he could have found it sooner. It was something he had never seen before and he was looking at a jaw infection instead of the inner ear. He had every reason to suspect a jaw issue. Like you said, I bet he will NEVER make that same mistake again, and will double check ear/jaw issues from now on. An awful circumstance but because of it he was able to save the next dog. We still love the vet and go to him even though it's a long drive after moving - he really did go above and beyond for the dog, doing tests and research on his own time that he didn't charge us for, even sending some to other states and overseas.
It's all a matter of finding the right veterinarian. Had he made the same mistake again (like the vet improperly euthanizing several horses) we likely would have not gone back.