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Software Testing – Bug vs Defect vs Error vs Fault vs Failure

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Software Testing defines a set of procedures and methods that check whether the actual software product matches with expected requirements, thereby ensuring that the product is Defect free. There are a set of procedures that needs to be in mind while testing the software manually or by using automated procedures. The main purpose of software testing is to identify errors, deficiencies, or missing requirements with respect to actual requirements. Software Testing is Important because if there are any bugs or errors in the software, they can be identified early and can be solved before the delivery of the software product. The article focuses on discussing the difference between bug, defect, error, fault, and failure.

What is a Bug?

A bug refers to defects which means that the software product or the application is not working as per the adhered requirements set. When we have any type of logical error, it causes our code to break, which results in a bug. It is now that the Automation/ Manual Test Engineers describe this situation as a bug.

  • A bug once detected can be reproduced with the help of standard bug-reporting templates.
  • Major bugs are treated as prioritized and urgent especially when there is a risk of user dissatisfaction. 
  • The most common type of bug is a crash.
  • Typos are also bugs that seem tiny but are capable of creating disastrous results. 

What is a Defect?

A defect refers to a situation when the application is not working as per the requirement and the actual and expected result of the application or software are not in sync with each other.

  • The defect is an issue in application coding that can affect the whole program.
  • It represents the efficiency and inability of the application to meet the criteria and prevent the software from performing the desired work.
  • The defect can arise when a developer makes major or minor mistakes during the development phase.

What is an Error?

Error is a situation that happens when the Development team or the developer fails to understand a requirement definition and hence that misunderstanding gets translated into buggy code. This situation is referred to as an Error and is mainly a term coined by the developers.

  • Errors are generated due to wrong logic, syntax, or loop that can impact the end-user experience. 
  • It is calculated by differentiating between the expected results and the actual results.
  • It raises due to several reasons like design issues, coding issues, or system specification issues and leads to issues in the application.

What is a Fault?

Sometimes due to certain factors such as Lack of resources or not following proper steps Fault occurs in software which means that the logic was not incorporated to handle the errors in the application. This is an undesirable situation, but it mainly happens due to invalid documented steps or a lack of data definitions.

  • It is an unintended behavior by an application program.
  • It causes a warning in the program.
  • If a fault is left untreated it may lead to failure in the working of the deployed code.
  • A minor fault in some cases may lead to high-end error.
  • There are several ways to prevent faults like adopting programming techniques, development methodologies, peer review, and code analysis.

What is a Failure?

Failure is the accumulation of several defects that ultimately lead to Software failure and results in the loss of information in critical modules thereby making the system unresponsive. Generally, such situations happen very rarely because before releasing a product all possible scenarios and test cases for the code are simulated.  Failure is detected by end-users once they face a particular issue in the software.

  • Failure can happen due to human errors or can also be caused intentionally in the system by an individual.
  • It is a term that comes after the production stage of the software.
  • It can be identified in the application when the defective part is executed.

A simple diagram depicting Bug vs Defect vs Fault vs Failure:

Bug vs Defect vs Fault vs Failures vs Error


Bug vs Defect vs Error vs Fault vs Failure

Some of the vital differences between bug, defect, fault, error, and failure are listed in the below table:

Basis Bug Defect Fault Error Failure
Definition A bug refers to defects which means that the software product or the application is not working as per the adhered requirements set A Defect is a deviation between the actual and expected output A Fault is a state that causes the software to fail and therefore it does not achieve its necessary function. An Error is a mistake made in the code due to which compilation or execution fails, Failure is the accumulation of several defects that ultimately lead to Software failure and results in the loss of information in critical modules thereby making the system unresponsive.
Raised by Test Engineers  The defect is identified by The Testers And is resolved by developers in the development phase of SDLC. Human mistakes lead to fault. Developers and automation test engineers The failure is found by the test engineer during the  development cycle of SDLC
Different types
  • Logical bugs
  • Algorithmic bugs
  • Resource bugs

 Defects are classified as follows:
Based on Priority:

  • High
  • Medium
  • Low

Based on Severity:

  • Critical
  • Major
  • Minor
  • Trivial
  • Business Logic Faults
  • Functional and Logical Faults
  • Graphical User Interface (GUI) Faults
  • Performance Faults
  • Security Faults
  • Hardware Faults
  • Syntactic Error
  • UI screen error
  • Error handling error
  • Flow control error
  • Calculation error
  • Hardware error
Reasons behind
  • Missing Logic
  • Erroneous Logic
  • Redundant codes
  • Receiving & providing incorrect input
  • Coding/Logical Error leading to the breakdown of software


  • Wrong design of the data definition processes.
  • An irregularity in Logic or gaps in the software leads to the non-functioning of the software.
  • Error in code.
  • Inability to compile/execute a program 
  • Ambiguity in code logic
  • Misunderstanding of requirements
  • Faulty design and architecture
  • Logical error
  • Environment variables
  • System Errors
  • Human Error
Way to prevent the reasons
  • Implementing Test-driven development.
  • Adjusting enhanced development practices and evaluation of
     cleanliness of the code.
  • Implementing Out-of-the-box programming methods.
  • Proper usage of primary and correct software coding practices.
  • Peer review of the Test documents and requirements.
  • Verifying the correctness of software design and coding.
  • Conduct peer reviews and code-reviews
  • Need for validation of bug fixes and enhancing the overall quality of the software.
  • Confirmation of Re-testing the process end to end,
  • Carefully review the requirements as well as the specifications.
  • Categorizing and evaluating the errors and issues.
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Last Updated : 06 Dec, 2023
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