See the ECG Tutorial section on QT interval to learn how this is measured. Multiple medications, electrolyte abnormalities (hypocalcemia, hypomagnesemia and hypokalemia) and certain disease states ...

How often do you see an ECG that is just a little off? Maybe the T wave is flat, oddly-shaped or inverted. Maybe the ST segment is coved, very minimally-depressed or shows some J point elevation.