What is the biggest mistake one could make in customer service?

1- Taking a long time to answer.

2- Asking the customer every single minute to make his voice louder.

3- Telling the customer there's no problem when there is a big problem.

4- Delivering a bad messages like, "I'm busy, I can't help you, I want to sleep @..@".

5- Chews a gum.

6- Answering when you are in a bad mood.

From customer point of view, he/she prefers to hear from customer service employee a brief, convenient and clear words concerned the merchandise for sale to feel the honesty and confidencde in getting benefits from this service

Don't make promises unless you will keep them.

not paying attention when a customer is talking

Interrupting , Or Telling Him There's No Problem Why You Make it An Issue ,!

In my point of view the 2 most important mistakes are 1-Taking it personal when discussing a problem with a client
2- Lying to the client or even promising something you know you can never fulfill ,this is when you lose their trust for good

ان نمارس الضغط عليه لاقناعه من اكبر الاخطاء التى يمارسها البائع لبضاعضته مع المشترى

making arguments or impose your own judgment over them.

as far as i am concerned the biggest mistake would be not greeting the customer properly, because first impression is the last impression.

Customers are as fragile as expensive glassware.
You break your glassware because of your mishandling or because of losing your concentration on your path.
Here in our example ; 1.
Mishandling can be mentioned as getting aggressive on any repeated or silly argument from the customer, not giving proper attention and trying to avoid customer's point and imposing your own etc.
Remember Rule number 9 a customer can never be silly or wrong.
Rule Number 10 if sometime he/she is, then See Rule Number 9.
2.
The losing of concentration is when you respond slowly, do not hear or understand what is the actual point or problem customer is trying to mention and doing some other work while handling a customer call etc.
REMEMBER! When a customer calls you, start thinking that you've glasses piled up in a try and you've to walk on a rough surface without seeing down.

Never underestimate a customer or his request.
Listen to what he has to say, or complain about.
Solve the problem immediately, or if can't promise him you will do asap.