Once you get over the nearly-uncontrollable urge to strangle them to death with the nearest cord, the stupidity of the situation sinks in: This asshole is not only bitching because, say, you put his change on the counter instead of his outstretched hand because he just did the same thing to you. He's also quoting a proverb and expecting you to comply with it like it's a universal law.

WRONG!

A proverb is a phrase recycled to the point that it's used in everyday conversations. Usually the person quoting a proverb will act smug like they have some little piece of wisdom that makes them superior and/or absolutely correct; at which point you have to explain to them that proverbs don't make you wiser or more accurate, but they sure as hell make you more annoying.

In the case of the proverb stated above ("The customer is always right"), the meaning is always taken out of context. It does not imply consumer infallibility-- because if that were the case they wouldn't ever be lost, confused, or smell like they just crawled out of the bowels of the nearest trailer park. It implies "If we don't make the customers feel ashamed and embarrassed... and instead let them wallow in their own delusions, they'll come back and continue to spend their money at our business."

But God forbid you say something to that effect; then they'll cry to the manager about your "rudeness." And your manager will stand there with a fake smile on her face saying, "Yes, okay; I'll have a talk with him." while simultaneously thinking, "Why is this old smelly mother-fucker talking to me? Why is he still talking? Why won't he go the fuck away? (Your name here), I will have my revenge for letting this bastard talk to me. Oh god, I'm going to gag if he doesn't get the hell away from me this second."