I don't mind ignorant people.
I don't mind people who are sure they're right.
But ignorant people who are sure they're right despite all evidence to the contrary get my angst burning something awful...

So a Californian couple came in and ordered two doppios. I proceeded to pull them some very nice doppio ristrettos. We always pull ristretto shots--there's no good reason to do otherwise. If a customer asks me specifically for a 2 oz doppio, or even a lungo, I'll pull it for him, but only after a significant conversation where I make sure that he understands that he's ordering an inferior product.

The Californian couple received their doppios (in paper cups!) and each said, "this is only a single."
I said, "No, they are doppio ristrettos."
They said, "No, they are singles."

I then explained what a ristretto is.
They said they knew what a ristretto is, and that this was only a single shot.
I got pissed, invited them behind the counter and pulled two more sets of shots; one a ristretto, the second an over-extracted but full 2 oz doppio. The still didn't believe me.

We argued for several more minutes. Then I pulled the Italian trump card: "In Italy they always pull ristrettos." (Not completely true true, but I wanted to end the conversation.)

The guy replied, "Well I've been to Italy..."
I said, "It's not up for debate--this is how to pull espresso. I'm not arguing with you, I'm telling you."
He said, "Spoken like a true Italian."
That softened me just a bit, though I haven't an ounce of Italian blood. They left amicably, though I'm fairly certain that they still didn't believe me.

I'm still offended. I'm still pulling ristretto shots. No matter how much customers want me to make them bad coffee (and they do), I hold the line.