I've frequently heard "the customer is always right" quoted as if it were undeniably and always true. However, while the maxim expresses the generally right attitude toward customers, sometimes customers are dead wrong.

Here are three common situations where this is the case.

1. When a customer is disrespectful.

Being a customer does not free you from the obligation to show courtesy to people who are not being intentionally disrespectful to you.

For example, if a customer in a fast food place screams at a server for making a mistake, the customer is in the wrong. Yes, the mistake should be corrected, but if the customer continues to yell, he or she should be ejected and asked to never return.

Same thing with customer support. Bozos who yell at otherwise-polite customer-support personnel are in the wrong, regardless of how frustrated or angry they might feel.

Look, somebody working in a Bangladore boiler-room for $1 an hour isn't responsible for the lousy engineering, cut-rate manufacturing, and cheapskate outsourcing that is his company's policy.

Now, if you had the CEO of that company on the line, by all means feel free to tear him a new one, because it's his fault that you've had a lousy experience. But it's wrong take out your frustration on the peons.

2. When the customer is dishonest.

Being a customer does not free one from the obligation to be honest. For example, if a cashier accidentally undercharges you and you're aware of the undercharge, you're in the wrong if you fail to point out the error.

Same thing if a cashier accidentally hands you too much change. Just so you know, cashiers typically are pay-docked if the till comes up short, so customers who keep the extra change aren't "sticking it to the 1 percent," but instead taking it from somebody who might be struggling to feed a family.

Another form of customer dishonesty is pretending to be a dissatisfied customer to get a discount, like when a diner puts a bug into a meal to avoid paying. While that sounds like something out of a situation comedy, I've actually seen this sort of thing happen with my own eyes.

I was eating in a local sushi restaurant, when a chiropractor I know (my ex-wife worked for him) came in with a group of people and, when his order arrived, complained loudly about what he'd been served and demanded that the restaurant comp him the entire meal.

According to "the customer is always right" rule, the restaurant should have comped him, right? Well, I found out from the owner (whom I knew) that this was the fourth time the doctor pulled this in four successive visits to that same restaurant.

The owner refused to comp him, at which point he threw a tantrum, right there in front of everybody, hoping they'd comp him just to get him to shut up. They didn't and he departed in a huff.  Good riddance, as far as I'm concerned.

3. When customers bait and switch.

Customers are also in the wrong when they accept the expertise of a salesperson or a retailer and then buy the recommended or displayed item on the internet.

It's different if you do your own online research. In that case, you've every right to search around for the lowest price.

However, if a salesperson spends time helping you decide and that advice has been valuable, you are ethically obligated to buy from that salesperson.

Similarly, if you're browsing the displays in, say, a bookstore and you discover a book that you plan to buy, you're ethically obligated to buy it at that store, even if you can get it cheaper on Amazon.

In summary, while customers should be treated from the start as if they're always right, when they behavior is disrespectful, dishonest, or disingenuous, that favored treatment should come to an immediate stop.