I worked in a Burger King in high school, and in summers they started putting me on morning shift because I was fast with prep. I'll always remember my first shift on counter.
An old man came in and the first words out of his mouth were "Who the hell are you?"
Me: "I'm Tim. What can I get for you this morning?"
Him: "What's this crap 'what can you get for me?' I come here every day, Monday through Friday. You should know what I want."
Me: "I'm sorry, sir. This is my first day on counter. I don't know what you usually order."
Him: "Where the hell is Judy?"
Me: "She's not working today. If you'll tell me what you'd like, I can place your order."
Him: "This is BULLSHIT!"
The day manager heard him from his office, came out and took care of him. His usual was a plain sausage biscuit and senior decaf.
Cranky old people are just a delight, aren't they? Working that job taught me all the things you shouldn't do when you're a customer.