A 58-year-old man with previous mitral/aortic mechanical-valve replacement, aortic root repair, and coronary disease developed severe left-ventricular dysfunction following AV-node ablation/single-chamber pacemaker implantation for management of atrial fibrillation. He then underwent an upgrade to cardiac resynchronization therapy with a defibrillator. To manage his heart failure better, angiotensin-receptor blocker therapy was changed to sacubitril/valsartan, after which symptomatic palpitations with T-wave oversensing occurred. The resolved T-wave oversensing and palpitations stopped upon discontinuation of sacubitril/valsartan and recurred upon rechallenge, requiring a switch back to valsartan monotherapy. Our report presents the first known case of T-wave oversensing due to sacubitril/valsartan.