Patient data:  54-year old man who recently underwent a major cardiac procedure.  At first glance, this ECG may not appear to be particularly unique but a closer inspection reveals something unusual going on here. 

HINT:  The telling clue can be found in lead II but you'll have to use calipers and think "outside-the-box".  3 days after this ECG, the patient reverted back to sinus rhythm but continued to display this one finding.  Good luck!