Paired lights for lit signs
You see it everywhere -- signs on buildings where a light has gone out. It is often amusing where a missing letter changes the name of the company in some silly way.
They spend fortunes on these signs, but bulbs are hard to replace. So why don't they make them with a special unit that has sockets for 2 bulbs, and switches over to the backup when the first one burns out? It's not actually that much more expensive, as you are going to pay for the 2nd bulb eventually (especially incandescent) though here you pay for it earlier.
To get fancy, you want a way for it to tell somebody that a bulb went out. A small data over power chip could constantly squirt simple low-data-rate packets down the power line, "I have a bulb that went out!" Something like the x10 protocol, which can be transmitted by a chip that costs pennies.
Could use that protocol for remote turn on/off as well if you wanted. You would wait for a few bulbs to go out before sending a worker out to replace them. That worker costs more than the spare bulb anyway.