1 {scene: The Mos Eisley cantina on Tatooine}
1 Wuher: Hey! We don’t serve their kind here!
1 Luke: What?
2 Wuher: Your droids. They’ll have to wait outside.
3 Luke: What? They’re droids! You don’t even need to serve them! They don’t drink!
4 C-3PO: Actually, Master Luke, I could really use a mojito. This desert heat is stifling.

Of course, there's a huge canonical backstory about why Wuher, the cantina owner, is so prejudiced against droids. Apparently battle droids killed his parents back during the Clone Wars. He also hates bounty hunters, and slave traders, and pretty much anyone, really. Perfect personality to be a bartender.