Interrupting - Never interrupt the customer or finish their sentences. Most customer service providers want to solve the problem as quickly as possible. This is normal and natural, but it will prevent you from defusing the customer's anger; this customer will argue, will become withdrawn and get even more upset.
Being afraid of not having all the answers - In order to listen well, you don't need to have all the answers. This bad habit can cause you to miss the real issues. Sometimes the customer is looking for understanding, not necessarily answers.
Forgetting that the customer is probably having this problem for the first time - You've had to deal with this issue thousands of times and it is anything but new for you, but remember that the customer is experiencing this for the first time, so respond accordingly.
Believing that you are better informed than the customer - This habit can cause you to dismiss the customer's message, to anticipate it or to interrupt with a premature resolution. Leave your mind reading talents at home... They're not always reliable!
Reacting too strongly - When the customer's words strike a nerve, we tend to stop listening. In our heads, we start to disagree with the customer.
Doing more than one thing at a time - Being distracted or doing something else while listening tells the customer that we are not paying much attention to what they are saying.
Justifying yourself - Do not make the mistake of justifying what went wrong or choosing someone to blame ("The guys in shipping..."). You will become defensive and the customer will feel like you are looking for excuses, which lessens your credibility and professionalism.