This case report describes a 69-year-old woman who developed thrombotic obstruction of a mitral bioprosthesis while receiving VA-ECMO with left ventricular venting after complex multivalve surgery. Delayed anticoagulation, prosthetic material, altered intracardiac flow, and postoperative bleeding likely combined to trigger thrombosis. The report highlights diagnostic difficulty during ECMO weaning and stresses individualized anticoagulation, echocardiographic monitoring, and careful unloading strategy selection.