A 91-year-old male patient with chronic obstructive pulmonary disease in the intensive care unit was issued a red blood cell unit. Shortly after issuance, the transfusion failed because of an occlusion in the bedside leukocyte reduction filter. A small clot was observed at the spike portion of the returned filter, but the transfusion failure persisted despite replacement of the filter. On the bedside inspection, a large, irregularly shaped dark clot, approximately 2×2.5 cm in size, was observed within the blood bag, obstructing the filter spike and preventing blood flow. The unit was retrieved and classified as unsuitable for transfusion, and the supplying blood center was notified. A replacement red blood cell unit was then issued. This case highlights the importance of careful visual inspections and prompt management of abnormal blood components to ensure transfusion safety.