A 47-year-old woman is presenting to the Emergency department, complaining of epigastric discomfort of several hours duration:    

• Suspect Lead error (incorrect placement of the V1, V2, V3  electrodes)
• Otherwise, a normal ECG        

The P wave is negative in lead 3 and positive in V1 and V2. This, in addition to the unusual sequence the QRS pattern, strongly supports the suspicion that V3 is actually  V1, V1 is V2  and V2 is V3. Incorrect placement of the precordial electrodes is a common recording error. The computer ECG  program incorrectly diagnosed “septal infarction”. (because of the abrupt decrease of the R wave voltage from V2 to V3)

A second ECG is recorded:

•   The previous ECG had been incorrectly recorded ( V1-V3 reversal)
•   This tracing is normal