As a business aphorism, "The customer is always right" is as stupid a saying as the personal-side one of "We're all in sales now," a bit of inanity that I tackled a while back.
And, not only is the customer not always right, he or she is not even deserving of unconditional respect.
Rather, the customer deserves reasonable respect. This degree of reasonableness is based on past interactions with repeat customers and current customer behavior.
One should never behave with out-of-control emotion of course, even if a customer does, and especially not if an actual or would-be customer is deliberately doing so to try to provoke a response. Rather, one should be calm, cool, and correct — and nothing more.
Treating every customer as though he or she is always right encourages bad behavior in general, and specifically, "alligators." Having seen them as a newspaper editor, a publisher, and, years ago, a divinity school student and ministry candidate, I've seen this in action.