We had two bad vets - they weren't horrible, just not thorough - but they almost killed two of our cats.
When Tuxie was first ill, the vet suspected it was leukemia, and put him on leukeran (chemotherapy) before getting biopsies done and test results back. That almost killed him, because as it turned out, he had an autoimmune disease, NOT cancer - and the drug for cancer made his body stop doing the things it needed to to fight the autoimmune disease. And I mean almost killed him to the point that he needed a blood transfusion to survive.
When Flowerbelle was finally sent home from almost a month in the hospital after she was first rescued (she was in really bad shape), she had a bad cough. She wasn't responding to the antibiotics, her eye wasn't healing - she was still a mess. That's when we found the #1 vet in the area. Her cough was NOT from an upper respiratory - it was from lung worm - something that had not been seen in this county in at least 20 years. She was so infested, the new vet said she was at most a few days away from "drowning" in them. We were SO pissed at the old vet - we'd been heavily involved in cat rescue for over a year at that point, and that he could send a cat home with us without having done that final fecal check - well - we were mad at ourselves for not double-checking him, but it is something we'd come to expect, because ALL the animals we brought in there were feral. The new vet said he'd never seen a fecal smear that packed with parasites, ever, in his 20 years of practice. He was so crazy about the whole thing, he made all the other vets and the techs come look at the microscope! He couldn't believe it. It took him some research to figure out what type of parasite it was, because he'd never seen it before. But that's the kind of guy he is - he will always do the research.
That's why when people post here and things look serious - or their cat is not getting better - or the cat keeps peeing outside the box despite a "no UTI" problem - I always recommend seeing another vet. Even the #1 vet can make mistakes, and it is always a good idea to get a second opinion if it's a serious disease diagnosis or kitty isn't getting better.