Power
The customer is always right, which means you are never right. You are at the mercy of Susan, who totally told you not to put any room for milk and you totally did. Then there's Trent, who is, like, usually chill about things like this, but almond milk and soy milk are, like, different animals, man, so why would he drink something that was, like, different, you know?
People are going to nitpick or claim they said something that they didn't or tell you to do something that is not exactly fair or right—and you are just going to have to do it. If you want a tip, you have to do it while apologizing for your own incompetence; they are obviously geniuses who are also incredibly attractive and talented...
...but you can secretly give rude people decaf.