We describe a case of potentially fatal undersensing of VF by a third generation ICD with predetermined automatic gain control. In this patient, ventricular sensing was optimal, as R wave amplitudes during sinus rhythm were at least 16 mV. Cyclical, high amplitude signals during VF elevated the sensing floor to such an extent that complete undersensing of subsequent lower amplitude local electrograms occurred. This led to bradypacing and complete ICD therapy failure. Therefore, high R wave amplitudes during sinus rhythm do not warrant flawless sensing during VF.