A good example is when you get a bicycle, you do the equation and realize that within two years you will have saved in on the cost of public transportation. But it's when you start using it you realize that suddenly you always end up sweaty arriving to work so you need to buy good ventilated outdoor clothes and then a little bit later the winter arrives and you realize you need the same type of winter clothes, and then your first tier brakes and you realize you don't have any tools, so not only do you need to buy a new tier but also the tools. And here the initial smart calculation has fallen apart long time ago.
Don't get me wrong. There's a lot of benefits riding a bike to work so it may still be very much worth it. But once you have this realization you can apply the same logic to anything you buy or thinking of buying which may helps prioritizing because there's just so many black holes you can have in your life.