Quoted from CactusJack:

Another problem I have seen by using LEDs in an EM game is what I believe to be caused by induced current. If you see an EM where someone has replaced all the lamps in the backglass with LEDs, you can see various non-lit LEDs flash very briefly when various things are happening in the game (like scoring etc.) Unlike an Incandescent bulb, LEDs can light extremely fast, and, as described above, can light with very low voltage and current. When a wire has to pass current, it actually creates a small magnetic field around it. The more current it passes, the greater the magnetic field. If you lay another wire parrallel to the first wire, the current in the first wire will induce voltage in the second (The basic principle of how an air transformer works). So, by turning on a coil in a EMs head you induce voltage in the lamp wires (like Ball in Play, match lights, Game Over) which are laying parrallel to them in the wire loom. And you get faint little random flashes of light in some, if not all of the feature lamps. This may not be very noticeable down at the PF, and also not by the person playing the game (except maybe during bonus collect). But its easy to see by on-lookers.