A patient with congenital complete heart block underwent implantation of a dual-chamber pacemaker. He presented to the emergency room with fatigue and was found to be in atrial flutter. Device interrogation revealed undersensing of 5 mV flutter waves at a programmed sensitivity of 0.5 mV. Due to undersensing, mode switch did not occur. This case illustrates apparently paradoxical undersensing of atrial flutter waves by a dual-chamber pacemaker and can be explained by a phenomenon known as "quiet timer blanking."