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Why is it that software system sometimes don't work correctly? We know that people make mistakes - we are fallible.

If someone makes an error or mistake in using the software, this may lead directly to a problem - the software is used incorrectly and so does not behave as we expected. However, people also design and build the software and they can make mistakes during the design and build. These mistakes mean that there are flaws in the software itself. These are called defects or sometimes bugs or faults.

When the software code has been built, it is executed and then any defects may cause the system to fail to do what it should do (or do something it shouldn't), causing a failure. Not all defects result in failures; some stay dormant in the code and we may never notice them.

Difference Between Error Mistake Fault Bug Failure Defect

What is an Error or Mistake?

Error is a human action that produces an incorrect result. It is deviation from actual and expected value. The mistakes made by programmer is known as an ‘Error’. This could happen because of the following reasons

  • Some confusion in understanding the requirement of the software
  • Some miscalculation of the values
  • Or/And Misinterpretation of any value, etc.

It represents mistake made by people and Mistake in the program leads to error.

What is a Bug?

A Bug is the result of a coding Error or Fault in the program which causes the program to behave in an unintended or unanticipated manner. It is an evidence of fault in the program. Bugs arise from mistakes and errors, made by people, in either a program's source code or its design. Normally, there are bugs in all useful computer programs, but well-written programs contain relatively few bugs, and these bugs typically do not prevent the program from performing its task.

What is a Defect or Fault?

A Defect is a deviation from the Requirements. A Software Defect is a condition in a software product which does not meet a software requirement (as stated in the requirement specifications) or end-user expectations. In other words, a defect is an error in coding or logic that causes a program to malfunction or to produce incorrect/unexpected result. This could be hardware, software, network, performance, format, or functionality.

What is a Failure?

Failure is a deviation of the software from its intended purpose. It is the inability of a system or a component to perform its required functions within specified performance requirements. Failure occurs when fault executes.


A Bug is the result of a coding Error and A Defect is a deviation from the Requirements.  A defect does not necessarily mean there is a bug in the code, it could be a function that was not implemented but defined in the requirements of the software.

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Lakshay Sharma
I’M LAKSHAY SHARMA AND I’M A FULL-STACK TEST AUTOMATION ENGINEER. Have passed 16 years playing with automation in mammoth projects like O2 (UK), Sprint (US), TD Bank (CA), Canadian Tire (CA), NHS (UK) & ASOS(UK). Currently, I am working with RABO Bank as a Chapter Lead QA. I am passionate about designing Automation Frameworks that follow OOPS concepts and Design patterns.
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