Understanding Absence of Error fallacy in Software Testing

Is it possible that software which is 99% Bug free, but is still unusable?

Answer to this question is YES, it is possible. This happens when software is tested thoroughly but for wrong requirements. Therefore this should be always kept in mind that just because at the time of testing, defects were not found in the software, doesn’t mean that the software is ready to be shipped. Before making a decision to ship the software, a question needs to be asked. Were the executed test cases really designed to search most of the defects? or were they designed to check whether the software matched the user’s requirements?
If the software which is developed is not used as it does not fulfill user’s requirements and expectations then finding and fixing the defects will not help.

Coming to Absence of error fallacy, let us understand this with the help of an example. 
Imagine, our company developed a school management software. Complete testing of the software is done and about 99% of the defects which were identified have been fixed and also retesting on them is done. This report has been submitted to management and this has given them pretty much confidence about the product quality w.r.t. defects.
Now when the demonstration of software is given to the client, he gives his feedback saying “Though it is called defect free, but still this is not what I required.AS I wanted a simple UI which can handle the user load.”

Now before receiving this feedback, everyone in the Testing team was very much confident and sure about the 'quality' of product (i.e. absence of errors) but in the end, it proved to be false (i.e. fallacy). This straightaway means that the system is not usable by the client as it does not fulfill his expectations.

Therefore, it is very much important to note that the Client's requirements and expectations are as important as the quality of product.

Happy Testing:)