I believe this problem is under appreciated in the use of the external (transcutaneous) pacer. Ineffective pacemaker discharges create an electrical artifact. As the deflection returns to baseline it resembles a QRS complex (false capture). To complicate things further, the skeletal muscle twitch caused by the pacer's impulse may be mistaken for a pulse. Here's a post from EMS 12-Lead which illustrates a case of false capture and provides some troubleshooting tips. Here's another post (from the 12-Lead ECG blog) which adds another troubleshooting tip: using capnography!