What would you say is the single biggest mistake companies make when speaking with angry or unreasonable customers and what can we do about this?

They get clever or smart, this is a mistake many make.  You don't know the IQ of the person you are speaking too, you don't know how long they have been in the business.  Even if you find that the toaster does not work because they have not plugged it in, don't gloat !!  Just say Thank you for calling, I am happy we managed to get to the heart of the problem, should you require further assistance sir, my name is XXXXX and I will be happy to take your call'.

Remember the customer is always right, every successful business uses this philosophy, always try to compromise with the customer, most of the time customers just need to vent, the best customer service agents are those who are good listeners.

True Customer is the King But remembers as rightfully said u need to be a key listener,and response should be instant,and be able to think of the blue...and make him feel you really care and empathise with the customer...and also if you can do something good make sure you do it for him.....