WARNING: This joke is slightly racist. However, it's not so bad because it doesn't really matter *which* two races you use, it can work with any.
A Mexican walks into a bar and sees a black barkeeper working there, so he says to him, "Oi, darkie, gimme a beer!"
The barkeeper looks at him and says, "Look, I'll get you your beer, but I'd appreciate it if you watch what you say in here."
The Mexican just gives him a dirty look, takes his beer, and starts drinking it.
5 minutes later, he's finished it, and yells "Oi coon, I want another beer!"
At this point, the black guy is furious. He yells at the Mexican, "Why are you saying that crap! How would you like it if you were working here and I started calling you slurs?"
The Mexican replies "I don't know to be honest. Why don't we swap places and find out?"
So the Mexican puts on the bar uniform, starts scrubbing dishes and serving customers. The black guy walks out, and walks back in casually dressed.
The black guy sits down at the bar, and yells "Oi, spic, gimme a beer!"
The Mexican looks at him and calmly replies, "I'm sorry. We don't serve niggers here."