What are some important concerns when placing ecg electrodes on a patient? why is it important that the ecg leads are placed correctly? what are the consequences of incorrect placement?

The ECG machine will read the electrical charges from the electrode and the location of the electrode will influence the readings.
Lead V6 used for examining the left part of the heart and that is why it was put on the leftmost side. 
If the electrode is not placed correctly the reading might be disturbed, leading to wrong examination result. The consequence of the incorrect placement would be the patient got wrongly diagnosed. The result might fail to diagnose existing disease or make a normal ECG become pathological.