When you just bought a new vehicle and you instantly buy a carb rebuild kit. Because even though it is fine right now, you know you will need it... eventually

You keep 2 screwdrivers in the glovebox. One to use as a key and one to use to use as a screwdriver. And you've used them both at the same time.

You have 2 vehicles with broken starters and only enough money to buy 1 starter. So instead of buying a starter for either vehicle, you spend your money buying a truckload of dirt and gravel to build a hill in your yard so you can roll - start them (my uncle actually did this)