You have an old house and everything is a bit outdated, but you're satisfied. You add a room air conditioner during one particularly hot summer. A few years later, when you have more money, you decide to add a central air-conditioning system. But you don't remove that room unit in the bedroom — why would you? It might come in handy and it's already there, bolted to the wall. Then a few years later, you have a catastrophic plumbing problem — pipes burst in the walls. The plumbers need to break open the walls and run new pipes, but your central air-conditioning system is now in the way, where some of their pipes would ideally go. So they run the pipes through the attic, the long way around. This works fine until one particularly cold winter when your uninsulated attic causes your pipes to freeze. These pipes wouldn't have frozen if you had run them through the walls, which you couldn't do because of the central air-conditioning. If you had planned all this from the start, you would have done things differently, but you didn't — you added things one thing at a time, as and when you needed them.