Source: Understanding customers by Ruby Newell-Legner
Though finding unhappy customers can be a tedious task. Generally there are three kinds of unhappy customers you will come across. Though the first and second kind of unhappy customers are easy to catch i.e. either they bring to your notice by way of feedbacks or contacting the customer service or by shouting about you on the internet (review websites, community forums & groups, etc.,). There is one another kind i.e. the Third Kind of unhappy customer whom identifying will be a mammoth of a task.
The third kind will never complain but vouch to never ever come to you again; they will migrate to your competitors and will not refer you to others. These are silent killers. This kind actually forms the larger part of your unhappy customers. It is difficult to track them, but in case of certain products or services which involve repeat purchases, you may find out these customers if your customer relationship system is in good place. You should be able to track them as to what they bought and did they return again for the same and if not then why. Unfortunately they form the major part of your unhappy customers (>90%)