I worked for a PC hardware outfit once during my college days.
The service engineer would carry a small bottle of oil with him. He would meticulously clean the printer, fix the ribbon, ensure it is running smoothly, fix any faults etc etc then while leaving just before packing his stuff he would dab a finger with the oil on the infra red sensor of the printer.
The result was that it would collect dust rather quickly over a few weeks and paper would get jammed!! Then the customer would have to call the technician more frequently than the fixed number of services allowed in the contract per year and pay extra ... :lol: Cheating .. yes
so beware of AMCs
Regards