A 56-year-old lady with arrhythmogenic right ventricular cardiomyopathy had a shock for ventricular trigeminy. The device diagnosed this as ventricular fibrillation because of its binning algorithm, which does not use a consecutive, or a proportional counter. A beat is binned as a fibrillation beat only if the current cycle length is in the fibrillation zone and the running average of the previous four cycle lengths are in the fibrillation or ventricular tachycardia zone. Reprogramming the device into a single detection zone will help prevent shocks in this situation.