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

Farhan Labib
2 min readNov 24, 2022


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.”


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.


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 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.


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.”


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.