Mix It Yourself
There’s not a lot a barista can legally do to get revenge on rude customers. Sure, they can swap decaf with regular coffee and vice versa, or they might make your drink super-duper sweet, but if you complain, the barista will have to make the drink from scratch, meaning they’ll have to deal with you for a few more minutes.
But this barista found a clever trick to get back at rude customers. Before serving the drink, they would intentionally forget to mix it up, leaving all the undissolved sugar crystals at the bottom. All you had to do was be a kind person to receive your drink just the way you wanted it.