Collision tumors, even uncommon, characterized by two distinctive (morphological, as well immunohistochemical) and spatially independent tumor components in the same location, are always puzzling for clinicians, pathologists and patients because they don’t fit in usual approaching schemes, neither diagnostic, nor therapeutic. Reviewing the specialized literature, until now, collision tumors have been reported in multiple locations such as the skin, esophagus, stomach, liver, kidney, bladder, adrenal gland or thyroid. We report a case of coexistence on the same site of a malignant tumor of ascending colon and a benign one emerging from peritoneal lining, thought initially by the surgeon to be right sided serosal carcinomatosis. But the histopatological examination reveals that those multiple serosal nodules were benign granular cell tumors collided with highly aggressive transparietal signet-ring colon carcinoma. These results put the patient's prognosis and therapeutic strategy in a different light than the clinical-imaging and intraoperative assessment.