Everyone should work a customer service oriented job at some point in their life. This could be in the food service or retail or hospitality or truly anything that requires the ideal that "the customer is always right." And that is simply to save you from being one of the many different customers that employees in this job field complain about.
For example...
1. The "may I speak to the manager" customer.
This customer does not care how long you have held your job or how much experience you have, they just want to speak with your manager. No matter how many times you recite corporate policies or apologize for the inconvenience, they just want to speak with the manger. And the worst part is that, you know your manager is going to be annoyed that you couldn't handle the customer's simple request.
2. And the opposite customer.
Surprisingly enough there is a customer that does the exact opposite. This customer doesn't care that you need manager authorization or that their request is not in your job description. All they want is for you to do what they are asking... now. Maybe they have authority issues.
3. The TMI customer.
This is the customer that comes in and tells you their whole life story. You ask them how many people are in their party and they tell you about their recent divorce. You tell them their total is 12.34 and they tell you about how their granddaughter is going to medical school. It is as if instead of "sales associate" your name tag says "your new best friend".
4. The loud laugher.
Even if you don't work in customer service, we all know this customer. You can hear them from all the way across the restaurant. They are making conversation with the table next to them and the table on the other side of the restaurant. They are shouting to their friends across the store and playing Marco Polo. And they need to leave.
5. The complainer.
It doesn't matter whether you give them the best table available or you give them a discount on their purchase, they have something to complain about. This is the customer that is know to go 0 to 100 real quick over the smallest details possible.