Open Mouth, Insert Foot

I'm used to moments like these, but they don't usually embarrass me like the situation today.

I'm at work, wiping down the condiment bar and tables, making sure there's enough creamer for customers. I glance back at the line forming at the register and see a guy wearing a T-shirt with big yellow writing. I see W_I_E GUY, and think it's funny that a white guy would have WHITE GUY written on a T-shirt. I rush to my bar position, seeing that Kim is dealing with an issue at the register, so I do what baristas are supposed to do and begin taking drink orders while the customers wait to pay. The situation went something like this.

Me: Hi! Can I get a drink started for you?
Customer #1: Yeah, I'll have a grande non-fat latte.
Me: And what's your first name?
C #1: Danielle.
Me: Thanks, Danielle. Kim will ring you up in a sec.

(To the next customer in line)
Me: Hi! What can I get started for you?
C #2: I'll have a grande white mocha without whip cream.
Me: Can I have your name?
C #2: Nicole.
Me: Thanks. Kim will ring you up in a bit.

(To the next customer in line--the guy whering the W_I_E GUY T-shirt)
Me (speaking loudly, even for me, because it's loud and he's a good six feet away): Hey, White Guy, can I get a drink started for you?

He slowly raises his head, and I see that his shirt says WISE GUY. Feeling stupid and embarrassed, I immediately apologize, explaing that I thought his shirt said WHITE GUY. Meanwhile, Danielle, who is African-American, looks at me with a shocked expression and begins laughing. Then I glance over at Nicole, who either Hispanic or Middle Eastern, and she's laughing as well. Danielle does not stop laughing, telling me she needed something like that after the day she's had. She also tells me she's going to tell all her friends. I feel myself getting red in the face and don't know what else to do but laugh with them. Luckily, Todd, as he now has a name and a drink, takes it well. In the end I couldn't charge him for his venti sugar-free vanilla non-fat latte.