Intracardiac sterile clots forming on pacemaker leads are an underestimated complication of cardiac device implantation and frequently present as an incidental echocardiographic finding. We report the case of a large pacemaker lead thrombus complicated by acute pulmonary embolism after few hours from its diagnosis in a 67-year-old man admitted for device implant revision. Although pacemaker leads clots seem to have minimal clinical significance and rarely cause clinical manifestations such as right-sided heart failure and pulmonary embolism, our case showed that large mobile thrombi on implantable device leads may have an unpredictable fate and can be rapidly cause of serious and potentially life-threatening complications.