Relays wear out and duplicate parts should be obtained to be able to service artwork sold. Repair concerns should be addressed at time of sale. It is perhaps smart to use a relay with a socket or removable plugs.
When a relay coil is de-energized a "back EMF" can be created. This is a high voltage spike that travels back through your relay signalling electronics, in our case our Arduinos. This can fry the electronics. A Flyback diode is placed across the lines to suppress the spike. http://en.wikipedia.org/wiki/Flyback_diode