One man had inoperable melanoma. It had spread in his peritoneal cavity. The surgeon didn't even try to remove it all. His wife was distraught. Some time later, he returned with symptoms of intestinal obstruction. Everyone assumed that the melanoma had caused it. When the surgeon opened his abdomen, to relieve the symptoms, all he found were adhesions. He couldn't find any trace of the melanoma. He biopsied several rather random places, but no carcinoma was found.
The other was a man with metastatic adenocarcinoma in the liver. It was demonstrated by X-ray and biopsy. He returned many months later with no symptoms. The X-rays showed no evidence of the carcinoma. There was nothing to biopsy.
In both cases, we feared that we had made an erroneous diagnosis. Just to reassure ourselves, we sent the X-rays, biopsies, et al. to a major US university, where I trained as a intern and resident. They agreed with the diagnosis and asked why we had sent so obvious a diagnosis for consultation. I explained that no evidence of the carcinoma could be found. Everyone was astonished.