1. The Complainer. "This drink is too cold." "This drink is too hot." "This drink has too much milk." "This drink doesn't have enough milk."
The secret: don't argue. Just remake the drink. You're not going to win the argument. You're going to make the customer more angry. Give them a new drink and a smile. It costs you $2 in ingredients. It saves you a negative review and lost goodwill.