Razors and blades
If you buy an inkjet printer, odds are the manufacturer lost money on your purchase. HP sells the deskjets for as cheap as they can... because they know they'll make a killing on the cartridges.
So, if you were HP, it would seem like the best thing to do is to be sure that people are using your printers, and keep using them for as long as possible.
Not so. I just called HP for help with a driver for my 18 month old printer. They won't help me on the phone... it's out of warranty.
Of course, if I buy a new one, the driver will still need help, and they will have lost money on my purchase of the machine that replaces the perfectly good machine on my desk.
Lesson 1: careful with those policies.
Lesson 2: razors should last a long time and be extremely well supported if you hope to sell more blades.