One pretty stupid thing I did was shouting at a customer at work...
I work at a Subway restaurant, and one night, me and my best friend were working together. It was busy as hell, and we were running out of bread and didn't have time to order more.
So I decided to call another restaurant and ask for bread from them, while my friend was serving the customers. One guy in the line started asking all the time stupid questions... "When will it be my turn?" "Hurry up!! I don't have all night!" "Why aren't there more people working?" and stuff like that. Then he yelled at me: "WHY AREN'T YOU MAKING SANDWHICHES INSTEAD OF HANGING ON THE PHONE?"
I put down the phone, and told him (in quite loud voice) that if he wants his f**king sandwhich, he has to give me time to get the bread.
But what I didn't know, that in the line, was a person working for the office. Few times a month, somebody from the office (we don't know them) goes on the restaurants, and sees how they're doing. And they don't let us know. And this person called my boss, and we got a low grade that month on the service-survey
