We report a fatal device-device interaction between a wearable automated defibrillator (WAD; LifeVest -LifeCor, Inc., Pittsburgh, PA, USA) and a unipolar pacemaker that occurred in an 18-year-old patient listed for cardiac transplantation due to his failing Fontan. The patient developed ventricular tachycardia that was initially detected by the WAD. However, large unipolar pacing artifacts and specific WAD arrhythmia detection algorithms caused the WAD to revert to nonrecognition of the arrhythmia, which lead to the patient's death. We identify likely causes of the failure and suggest methods of preventing such occurrences in the future. (PACE 2008; 31:912-915).