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Collision tumours are uncommon lesions in which two histologically distinct neoplasms coexist at one anatomical site, creating diagnostic difficulty when one component predominates clinically or histologically. A 70-year-old man presented with a large exophytic pigmented tumour on the lower abdomen that had been present for approximately 20 years and had enlarged rapidly during the preceding 5 years. He was brought to the emergency department because of tumour bleeding, progressive anaemia, poor nutritional status, and difficulty with mobilisation. Contrast-enhanced computed tomography during emergency systemic assessment revealed multiple pulmonary, hepatic, and bilateral lymph node lesions suspicious for distant metastases. Because urgent local control was required, no preoperative incisional biopsy was performed, and complete excision with immediate reconstruction was undertaken as a therapeutic excisional biopsy. Initial histopathological and immunohistochemical evaluation showed basal cell carcinoma with BerEP4 positivity. However, the metastatic pattern was inconsistent with basal cell carcinoma, prompting biopsy of a pulmonary nodule, which demonstrated malignant melanoma. Because primary pulmonary melanoma is extremely rare, the abdominal lesion was re-evaluated. Additional sampling from previously unsampled areas identified a MelanA-positive atypical melanocytic component adjacent to and partially intermingled with the basal cell carcinoma, confirming a cutaneous collision tumour. The distant lesions were interpreted as metastases from the melanoma component. BRAF V600 mutation analysis was negative, and nivolumab plus ipilimumab was initiated. However, the disease progressed rapidly, further systemic therapy was discontinued, and the patient died shortly thereafter from cancer-related complications. This case highlights how sampling bias and clinicopathological discordance can obscure aggressive malignant components in cutaneous collision tumours. In large, heterogeneous, or clinically aggressive cutaneous tumours, systematic multi-site sampling and prompt clinicopathological reassessment are essential when the initial pathological diagnosis does not fully explain the clinical behaviour.