Now let's look at the flipside. Just as the customer has the right to expect that he will get his money's worth when doing business with you, you have the right to expect that your customer will not demand things that are beyond the scope of realistic expectations (or the contract). If a customer orders hamburger, he shouldn't expect it to taste like steak unless you have advertised it as such. If a customer brings you a cotton shirt to launder he should not expect a silk shirt in return. It's when the customer's expectations get out of sync with what should realistically be expected that you will have problems.
We have all had customers who expected far more than was their due: customers who were unreasonable, overly-demanding, condescending, hard to please and sometimes, even dishonest in their dealings with you. When a customer's reasonable expectations become unreasonable demands you must decide whether or not that customer is doing more harm to your business than good.
So here is the line in the sand between the "customer is always right" and "sometimes you have to give the customer the boot" - if a customer crosses the line from being an asset to being a detriment to your business, you should consider giving that customer the boot.