Transvenous pacemaker and/or defibrillator lead placement into the left heart chambers is rarely done. Approximately a third of such cases reported in the literature presented with signs of thromboembolism, mostly neurological deficits. We describe a patient who presented with a cerebrovascular accident three months after inadvertent and unrecognized lead placement into the left atrium and ventricle through a sinus venosus atrial septal defect. Implant techniques to avoid this complication are discussed.