One of the problems I have when dealing with angry cutomers is trying to make someone happy and satisfied when they are 100% in the wrong. You know the customers I am talking about. The ones who damage products by abusing them or providing wrong information or refusing to listen to a salesman or anyone else during the sales process.
These can be difficult situations because often times you cannot take a product back and get credit or even get it repaired for free when someone abuses it or drops it. In other cases you cannot get the customer to understand that they were the original cause of the problem and that they have to change sometthing going forward or they will continue to have trouble.
You have to tread lightly in these situations because saying the wrong thing or using the wrong words can cause a bad situation to get worse very quickly. But what I have found that usually works is softening the message by giving the customer a "way out" without making them feel embarrassed and without being confrontational.
So instead of saying, "Look, you cannot use this product that way. You have to use it this way or you are going to have a problem." I might say "This is something I have run into several times before. In fact, I did it once myself. To make it work correctly all you have to do is this."
That delivers the message without making the customer feel foolish. It allows them to retain their dignity and does not cause them to feel confronted or accused. This helps put them in a positive frame of mind which makes them eaiser to communicate with.
Try to never back your customer into a corner with no way out. Whenever you do this they are far more likely to dig in and continue to fight just to save face and avoid embarrassment. Take away the confrontation, make them feel good and give them an easy way out to accept what you are saying and you should be able to resolve more problems in far less time.