A 64-year-old man was implanted with a cardiac resynchronization therapy defibrillator (Protecta, Medtronic Inc., Minneapolis, MN, USA) for the primary prevention of sudden cardiac death due to ischemic cardiomyopathy and refractory heart failure. The patient presented following a shock from his device during physical activity without any preceding palpitations or syncope. Device interrogation revealed one episode labeled as ventricular fibrillation (VF) which appeared to be terminated by a single shock. Table I summarizes the detection zone. The snapshot of the episode (Fig. 1) reveals a stable atrial high rate with a cycle length (CL) of 460 ms and intermittent “train track lines” representing mainly two populations of V-V intervals with CLs oscillating between 150 ms and 300 ms. Close inspection of the corresponding electrogram (EGM; Fig. 2) demonstrated intermittent double counting of the T waves (T-W) that corresponded to the intermittent “train track line” pattern seen in the snapshot. The EGM revealed one-to-one atrioventricular conduction with a variable ventricular rate, ranging between 430 ms and 480 ms. T-wave oversensing (TWOS) resulted in double counting of ventricular events, which in turn led to V-V intervals (FS) falling within the VF detection interval (VFDI). All atrial events following FS events due to TWOS were labeled as atrial refractory (AR) as they occurred within the postventricular atrial refractory period. Occasional tracking of atrial sensed events was noticed following “blanked or filtered” T-W. Figure 2(A) demonstrates FS events