Eight months after ICD implantation with an electrically active case the patient presented with ICD system sensing failure. Upon reoperation, we found an insulation defect of the proximal sensing lead and the generator case showed arc marks suggesting a short circuit between the sensing lead and case. The generator was replaced, the original sensing lead insulated, and a new sensing lead inserted. Bench testing of the explanted generator showed a damaged internal circuitry. Proximal lead insulation defects combined with electrically active cases may result in damage of the case. The potential damage of internal circuitry warrants generator replacement.