Undifferentiated cancer is a rapidly progressing cancer with poor prognosis. Sometimes, it is diagnosed at an advanced stage, and its origin is difficult to detect. A very unusual cancer was revealed by autopsy. The patient was an 83-year-old survivor of colon cancer, melanoma, and laryngeal cancer. He had been under watchful course observation after survival from laryngeal cancer but suddenly died due to aspiration pneumonia. The autopsy revealed undifferentiated cancer infiltrated the entire body, which was misdiagnosed with positron emission tomography (PET)/CT scan and MRI. The origin of this cancer was a mystery even with vigorous pathological evaluation. The patient was told that his previous cancers were all healed; however, undifferentiated cancer progressed rapidly to the entire body, just like "cancer diaspora". This report highlights the limit of diagnostic imaging tools for aggressive cancer, sounding the alarm for clinicians to look beyond old presumptions.