At my workplace, we have been told that "The customer is always right" is not to be tolerated.
What I have been taught is that "The customer is always first".
What this means is that you can correct a customer and you can tell them that they are wrong, and that the store down the street does not, infact, have that model for half the price. They have one that looks like it, but is not the same.
However, if there is a customer there, unless you are serving someone else, you are to drop what you are doing to serve them. They are not right, but they are the most important thing around you.