Error vs Defect vs Failure in Software Testing

Error or Mistake vs Bug or Defect or Fault vs Failure

In this tutorial, we will study the difference between Error, Mistake, Issue, Fault, Defect, Bug, and Failure. We have already discussed the above terms in previous tutorials. Here we will focus on the difference between them. All the above terms are generally used when the system deviates from its expected functioning and behaves abnormally. These terms are often used interchangeably and are interlinked with each other, but there is a slight difference between some of the terms.

As a newcomer, some people might get confused between error, bug, defect, issue, fault, and failure. When a tester identifies an abnormal behaviour of the system, it may call it a bug or error or defect. Sometimes, we call it an issue in the system. Hence, multiple terminologies are used for a particular thing, and new testers and engineers might confuse the terms.

Therefore, it is very important to thoroughly understand each terminology as we may encounter these terms in daily workflow while testing the system. To avoid confusion, we will study and learn the difference between the terminologies.

What is an Error or Mistake?

An error is a programming or coding mistake made by the developers while developing the software. In other words, it is a human mistake which results in the variation between actual and expected outputs. Errors are usually generated in the source or internal code of the application. It can be caused by misunderstanding the requirements, misconceptions about the working module, miscalculations, or misinterpretations of some values in the code.

Mistake – An error is also acknowledged as a mistake. A mistake is generally found in the source or internal code of the application. It is a human error.

There can be multiple types of errors generated in the program, such as syntax errors, logical errors, interface errors, database errors, performance errors, data handling errors etc. Errors are identified or detected when the code gets compiled and fails to meet the expected result.

What is a Defect?

A defect is an error or flaw in the system which deviates the product or application from performing its intended functions and results in incorrect or unpredicted outputs. In other words, the defect is a coding or logical error which makes the system inefficient in working properly as per the client's requirements. The reason behind the occurrence of defects could be compatibility with hardware and software, network issues, performance issues, functionality, or format. Due to the defect, the system or application will not work as per the requirements and specifications and will provide unexpected results. Defects are usually identified when the application is under the testing phase or when it goes into production, and then the defects are identified by the customers.

What is a Bug or Fault?

Bug – A bug is acknowledged as the informal name of a defect that makes the system operate incorrectly. In other words, when a mismatch is found in the system or application during testing, it is called a bug. Bugs are generally identified while performing Unit testing.

Fault – A defect is also acknowledged as a fault. It is usually known as the result of errors occurring in the system. A fault can be generated in a system for reasons such as incorrect or invalid steps, data definition, process, and lack of resources.

What is an Issue?

The term issue is a mix-up of all the terms such as bugs, errors, mistakes, etc. Therefore, it can be anything from a change request, incorrect or missing requirements, newly added tasks, etc. Generally, it is a term used by clients and customers when they find something inappropriate with the software being developed. It is a non-general term used when the stakeholders or clients are not sure about the problem and from where it is coming.

What is a Failure?

When there are multiple bugs in an application, the system fails to perform its expected functionality which leads to a system failure or crash. In simple words, when an issue or defect is identified by the end-user or client, it is called a failure as the system cannot meet the software's quality.

Sometimes, a system is not compatible with the other devices, which results in system failure. There could be possibilities when a single issue or bug can lead to one system failure or multiple failures. A failure can happen due to several reasons such as environmental issues, load and performance of the system, usability power of the system, etc.

The general flow of a failure is, first, an error or mistake made by a human, which leads to the identification of a bug or defect or issue. In contrast, testing, at last, when those bugs and defects remain unresolved, may result in a system failure.

Difference between Error, Bug, Defect, Fault, and Failure

The difference between the above terms is mentioned below in the table:

Attributes	Error/ Mistake	Bug	Defect	Fault	Failure
Definition	It is a human mistake which results in the variation between actual and expected outputs. Errors are usually generated in the source code of the application.	A bug occurs when there is a mismatch in the system under test. It is also called the informal name of the defect.	A defect is a coding or logical error which makes the system inefficient in working properly as per the requirements.	A fault is known as the result of errors that occurs in the system.	When an issue or defect is identified by the end-user, it is called a failure or system failure.
Identified By	It is generally identified by the Developers.	It is generally identified by the testers or test engineers.	It is also identified by the testers while testing the application.	It is generally identified by developers or testers.	It is generally identified by the customers or end-users.
Occurrence	It occurs due to misunderstandings and misconceptions related to the requirements and mistakes made in the source code.	It occurs due to shortcomings or mismatches in the system.	It occurs due to the coding or programming mistakes made by the developers.	It occurs due to loopholes or inconsistencies in the program.	It occurs due to single or multiple defects, issues, or bugs in the system.
Reasons behind	Reasons behind the errors are: Mistakes and miscalculations of some values. Misinterpretations of requirements and designs.Invalid or incorrect loops and syntax.	The reasons behind the bugs are: Wrong code Missing or ignored code Extra code	The reasons behind the defects are: Errors in coding and logic. Providing erroneous input data. Wrong implementation of the internal structure and design of the application.	Reasons behind the faults are: Inappropriate or incorrect data definitions, processes, and steps. Lack of resourcesIrregularity and inconsistency in the program.	Reasons behind the failure are: Human errors Usability of the system. Environmental issues.
Types	Types of errors are: Syntax errors Data handling errors Interface errors Arithmetic errors	Types of bugs are: Logical bugs Resource bugs Algorithmic bugs	Types of defects are: Based on severity Blocker Critical Major Minor Low   Based on priority Urgent High MediumLow	Types of faults are: Security faults Performance faults Software faults Hardware faults	Failures don’t have any types.
Ways to prevent	To prevent errors in the system, we should enhance the software quality by performing code reviews and inspection activities and understanding the requirements clearly.	To prevent bugs in the system, we should follow test-driven development and evaluates the source code systematically.	To prevent defects in the system, we should follow proper and correct development techniques, and peer review and evaluations must be done once the code is developed.	To prevent faults in the system, we should analyze the readiness and correctness of the system’s internal structure and design.	To prevent failures from the system, we should perform testing thoroughly with all the test cases being retested to ensure the system is 100% defect-free.