I know of several cases. My son was told he had a stomach upset, when it was appendicitis. Luckily I followed my instincts and took him to A & E, and they operated just before it burst.
My daughter was treated as a hypochondriac after having her gall-bladder removed, because she said she was in pain, feeling ill and had no appetite. A week later she was in intensive care, with septicaemia and peritonitis, and almost died, after an emergency operation. Her bile duct had been perforated during the first op, and her abdomen had been filling up with corrosive bile.
I was diagnosed with angina, some years ago, after having tests because of strange occasional feelings in my chest, neck and jaw. Eventually, after querying their diagnosis, I was sent for tests and told that I didn’t have angina at all. The symptoms still occur, but rarely, and I still don’t know what causes them, but my heart is fine!