I have been working in retail for 6 years now, and my current job as CSR in a gas station for a year and a half. I'm a very patient person, but I don't take disrespect well. This is a story of one of the worst customers I've ever had.

A guy comes up and orders a pack of cigs that we do not have in stock up front, so I go to the back to try to find them in our vast cage of cigarette cartons. While this is going on, my assistant manager (who is one of the most inept and conniving people I know; another story for another time) decides to take care of another customer since there is a line forming. The woman orders two packs of Winston Lights. By the time my assistant manager gets her cigarettes, I come back to tell the guy I was taking care of that we were currently out of stock. He orders another pack of cigarettes and as I go to scan them, the woman my assistant manager was working with (We'll call her POS for obvious reasons) exclaims "Wait! That's my order!" Now, the guy I was working with had some other things like a Red Bull, and the current total was $5.34. I looked at POS and said "Are you sure?" My assistant manager says "No, those are his, I haven't scanned your cigarettes because [my name] came back." So POS huffs and puffs because she was proved wrong and complains that she's late for work. The guy's transaction finishes, he takes his receipt and leaves. POS approaches the counter and I confirm with her "These are your cigarettes right?" and hold up the Winstons. POS loudly says "IF I CAN GET 'EM!" Right then and there I was ready to pick her up and throw her out of the store myself. But I decided to "kill her with kindness." I turned on my full sweet, patronizing voice, saying, "Oh, of COURSE you can get these!" I scan them, and POS swipes a card. Since we have separate credit and debit buttons, I asked her which one she was using. She snottily said "Debit!" So I said "Debit it IS" and hit the button. Her receipt prints and she snatches it out of my hand. As she turns to leave, I scream to her "AND YOU HAVE A GREAT DAY!" By the way, there were six other customers in line, so her little tirade (as well as my responses) was witnessed by multiple people.

Tl;dr: Don't act like a child, because I'll treat you like one.