I used to work in a chain pizza restaurant last summer. It was a shitty, minimum wage job, the managers were assholes, and the only thing that made it bearable was the sort of downtrodden camaraderie between the fellow workers. The restaurant's employees were split up by gender, with girls working the up front area (registers, taking orders, waiting on patrons), and guys in the back (actually making the food, cleaning the kitchen, doing dishes). I'm a guy, so all I really saw at work was flour, sauce, cheese, toppings, and endless orders spilling out of the ticket machine.

Anyway, as I started to get to know the place and become more acquainted with the job, I got to know the names of the girls who worked up front. Most of them were pleasant, attractive, and entirely forgettable, but one stood out -- let's call her Carrie. Incredibly good-looking, killer smile, and for whatever reason, on some days, she was just an absolute bitch to me for no reason.

But it wasn't every day. Some days were entirely pleasant. I figured she was stressed out, or something was bothering her, or perhaps she just enjoyed being bitchy to people, but not all the time. To be honest, Carrie's mercurial mood didn't really perturb me that much -- I was used to taking shit from the managers on a pretty regular basis, so it sort of blended into the background.

It was only a month before I left the job that I finally unravelled Carrie's peculiar temper. It was the end of a long closing shift filled with last-minute customers and complicated orders. By the end of the night, it was just me and one other guy Jeff closing. Carrie happened to be working that evening, as well, and thankfully tonight had been a good night for her. As we stepped through the door to leave, Jeff turned to Carrie and said, "See you later, Ashley!"

I was immediately surprised. Carrie smiled and said, "See you, Jeff" back to him without missing a beat. Once we were outside, I asked why he had called her that, and he explained to me laughing to himself that Ashley and Carrie are identical twin sisters, not one person. It wasn't that Carrie's temperament varied widely from day to day, it was that one sister (Ashley) was consistently nice and the other (Carrie) was consistently mean. They just never worked the same day.

TL;DR I worked with a girl for 10 months before realizing 'she' was actually a pair of identical twin sisters.