The electrocardiographic patterns produced by ECG lead misplacement are summarized in this paper.
Chest lead misplacement is pretty easy to spot. The most common limb lead misplacement is reversal of the arm electrodes. This pattern is well known and often picked up by computerized ECG analysis. Other limb lead misplacement patterns can be more tricky. Here is a summary of the more common ones:
Arm electrode reversal
Lead I is inverted.
Arm electrodes reversed with leg electrodes
Lead I is isoelectric. (This illustrates the principal that whenever you see electrical silence in a bipolar limb lead, it means both of that lead's electrodes are connected to the legs. It reflects the fact that there is no potential difference between the legs).
Right arm right leg reversal
II is isoelectric for the same reason as above.
Left arm right leg reversal
III is isoelectric.
The authors conclude:
A summary of the footprints of ECG lead malposition should be readily available for those who perform ECGs, those who interpret the tracings and those responsible for clinical care.
Well, here you have it. Much more detail is contained in the full text.