Have you ever noticed how some people, when they are a customer, feel like they have a license to be a jerk? These are the people who might seem nice enough until the waitress or cashier makes the smallest mistake, then they shred the person. I think they feel it's their duty to reprimand them, as a parent would do to a child.
When you are a customer, remember to be kind to the person providing you a service, especially if they make a mistake. You can choose to be nice instead of mean. Forgive their mistake and ask them to correct it. When a clerk makes a mistake, I am kind and they always thank me for my patience. I tell them that there are plenty of people who would be unkind so there's no reason I need to be.