My favorite one was a takeover at a house we prewired but ADT got the finish. The homeowner got fed up with them and when his contract was up he called us.
ADT had installed a complete wireless system. they used the keypad and siren wires but everything else was wireless, placed were we had intended to put wired devices.
5816's on the door slabs with the magnets on the jambs, a few inches away from the recessed contacts we installed during the prewire, wireless motion detectors covering the coil of wire in the corners were we intended to put dual-techs and wireless smoke detectors screwed onto the back boxes we put up when we ran the fire wire.At least I made the doors wired. Boss said not to replace the other stuff unless the sub wanted to pay for it so the motions and smokes stayed.