The customer is always right. Bullshit on that one.
Sorry, I have to step up here... but the real
meaning of this term, as told to me by my work experience employer when I was 15, is that the shopkeeper/service provider needs to remember that the customer has a point of view, and we need to respect that. It doesn't mean that the customer should always get free stuff if they demand it, for example, just that we need to figure out WHY they think that, and try to find a way to resolve things peaceably.
This, unfortunately, has been taken to the extremes by customers who've heard it, and now it tends to be used in an "I'm always right, you're always wrong" kind of way.