Pacing on the tail of paced QRS: What is the mechanism?

An electrocardiogram on the day following the implantation of a dual-chamber pacemaker revealed two closely coupled pacing spikes at 100ms intervals with the latter being on the trailing edge of the paced QRS complex. Differential diagnoses of such closely coupled pacing pulses include atrial lead displacement, lead reversal on the pulse generator, a ventricular premature complex in the post-atrial ventricular blanking period, and backup pacing pulse following a loss of capture. The critical analysis of the pacemaker timing intervals led to the mechanism and aetiology of the problem.