The Confusion: Error vs. Fault vs. Bug vs. Defect vs. Failure

Farhan Labib
2 min readNov 24, 2022

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Most of these terms — error, defect, fault, failure, and bugs — are interchangeable, although there are differences. In short- “A mistake in coding is called an Error, an error found by a tester is called a Defect, a defect accepted by the development team then is called Bug, the build does not meet the requirements, then it Is Failure.”

Error:

The problem in code leads to errors, which means that a mistake can occur due to the developer’s coding error as the developer misunderstood the requirement or the requirement was not defined correctly. For example, a developer may misunderstand a design notation, or a programmer might mistype a variable name — leading to an Error. It is the one that is generated because of the wrong login, loop, or syntax. The error typically arises in software; it leads to a change in the program’s functionality.

Fault:

A fault is introduced into the software as the result of an error. An anomaly in the software may cause it to behave incorrectly and not according to its specification. A fault may happen in a program because of the following reasons:

  • Lack of resources
  • An invalid step
  • Inappropriate data definition

If various components of the application code rely on each other, a fault may cause problems in multiple components. It causes a warning in the program. If left untreated, it may lead to failures in the working of the deployed code. A minor fault can result in a high-end error.

Bug:

Bug is the terminology of a Tester. A bug is a flaw in a software system that causes the system to behave in an unintended manner. A programming error causes a program to work poorly, produce incorrect results, or crash — an error in software or hardware that causes a program to malfunction.

Defect:

It can be simply defined as a variance between expected and actual. ISTQB states a defect as “A flaw in a component or system that can cause the component or system to fail to perform its required function, e.g., an incorrect statement or data definition.”

Failure:

A failure is the inability of a software system to perform its operations within the specified performance benchmark. As per ISTQB, “a defect, if encountered during execution, may cause a failure of the component or system.”

In other words, if an end-user detects an issue in the product, that particular issue is called a failure.

Possibilities are there one defect that might lead to one failure or several failures.

For example, in a bank application, if the Amount Transfer module is not working for end-users when the end-user tries to transfer money, submit button is not working. Hence, this is a failure.

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