A 79-year-old man with a pectoral implantable cardioverter defibrillator (ICD) system underwent periodic defibrillation threshold testing 18 months after implant. Attempted delivery of a 15-J shock caused a light flash above the pocket and a loud "pop." High-voltage lead impedance was <20 ohms. Pocket exploration revealed insulation abrasion of the high-voltage portion of the single-coil right ventricular lead. The outer shield of the active can pulse was perforated and scorched due to arcing. Device analysis confirmed a shorted transistor in the high-voltage output circuit. Unsuspected physical contact between high-voltage electrodes of opposite polarity within the pocket can cause catastrophic ICD system failure.