Double‐lumen subclavian catheters have recently been released for use as a temporary blood access for extracorporeal circulation. Initial reports indicate that once properly inserted, these catheters are safe. However, massive mediastinal hematoma that occurred 3 days after insertion and uneventful initial dialysis with a double‐lumen subclavian catheter has been documented. The patient experienced excruciating pain immediately after the onset of the subsequent dialysis and was found to have developed a massive mediastinal hematoma. It appears that at some point after the completion of the initial dialysis, the catheter tip eroded the vein and entered the mediastinum. As a result, the proximal (arterial) perforations were inside the vein, allowing blood to freely enter the circuit and be forcefully pumped into the mediastinum through the distal (venous) perforations near the tip. Prompt recognition of the problem and immediate discontinuation of dialysis prevented a potential fatal outcome in this case.