The way someone treats people says a lot about who they are as a person. How a person treats a worker in retail or the food industry, however, speaks way louder. I've worked in the food industry for almost 4 years, in various restaurants and forms, and let me say people are mean.
I'm not sure what exactly it is that makes people so mean to retail/food workers, but it's vicious. Things happen that are beyond our control in these situations. Restaurants get busy and stores carry items that go out of stock. This isn't the employee's fault, and people really shouldn't take it out on people who just work there. I say all this to say, please be nice to the workers you encounter at restaurants and any retail stores.
It's important to just be nice to everyone in general, but especially these people. We deal with a lot of people and a lot of crazy things happen in just one shift. The last thing we need is for someone to scream at us because it's busy and it's taking 5 minutes longer to get your food. It isn't the workers' fault that the shirt you love isn't in stock.
Things happen that people can't control. At the end of the day, you will be okay if something goes wrong and you didn't yell at the worker. The main takeaway here is please please PLEASE be nice to the people you encounter at restaurants and the people that check you out. Tip your server, be nice.