I enjoyed this article from Zakk Fleischmann. In it, he talks about how spending $5 for a Starbucks coffee is a "leak" and how this trickles into other areas of life.
Poker players have this concept called a leak: a blind spot that causes you to consistently make an error. As an example, a very common rookie leak is to only raise (put in more money) when you have a good hand. You can see why this would be problematic: after a few rounds, an observant opponent will be able to tell when you have a good hand and when you don’t.
There’s no question that getting an extra coffee is a small leak. But what starts as a $5 coffee on your walk to work can quickly become lunch out, with a drink or a side, and/or ordering dinner out instead of cooking what’s in your fridge. Yes, the first leak was only $3. But that leak grew throughout the day and now its $80 ($5 for coffee, $15 for lunch, $50 for dinner, plus $10 to have it delivered).
This reminds me of the idea of saying no to yourself. If you don't practice this, it will be hard to do so in any situation (this is why fasting is an important discipline). And once you say yes to one temptation, it's easier to give in to your desires in other areas.