Difference between Bug, Defect, Error, Fault, Failure

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Difference between Bug, Defect, Error, Fault, Failure

Sameeksha Medewar
Last updated on November 5, 2022

    We know that software testing is a well-structured process of assessing software products to identify and fix defects or bugs before releasing them to end users. Also, it ensures that software products under development are in compliance with the specified requirements and go well with customers’ expectations.

    Moreover, software testing entails detecting deficiencies and missing requirements in comparison to the actual requirements. You can make your software product excellent enough to meet consumers' expectations when you understand what requirements your software product is missing.

    Along with bugs, defects, and errors, software testing also spots faults and failures. All of them occur when there is a discrepancy between the actual outcome and the expected results. Often, there is confusion between all these terms, bugs, defects, errors, faults, and failures among novice software testers or students.

    This blog post aims at explaining to you the difference between bugs, defects, errors, faults, and failures associated with SDLC to have a better understanding and no confusion between these terms.

    So, let us get started!

    What is a Bug?

    In software development, a bug is the most common term you would often encounter. It is generally a defect or issue that leads the software product to behave unintentionally. It makes the software product work in such a way that end users did not expect, or developers did not intend to.

    When there is any logical error, it breaks the source code, which results in a bug. This is what automation or manual test engineers refer to as a bug.

    Whether it is a small and manageable bug or a complex bug, it has an adverse impact on the performance of the software product. The only solution to make the software product behave as expected is to fix bugs, whether they are simple or complex, immediately.

    The most typical and worst kind of bug is a crash. We can refer to a crash as a situation where a specific software comes to a halt and shuts down automatically midway through usage.

    What is Defect?

    In software testing, a defect is a situation that arises when there is a discrepancy between the actual results and the expected results and when the software product does not work in accordance with end users’ expectations.

    Basically, it is an issue in the source code that affects the entire software product. It occurs when software developers make a minor or major issue while writing the source code. When testing professionals conduct software testing on the software product, they spot defects.

    Defects make the software product inefficient and unable to perform the desired task and do not allow it to meet the criteria specified by customers.

    So, you can notice that a defect and a bug in software testing have very little difference, and both need to be addressed before deploying the software product to the production environment.

    Here are some common sorts of defects that testing professionals may come across while assessing software products:

    • Arithmetic Defects

    As its name suggests, an arithmetic defect is a defect present in an arithmetic expression, or it is an inability of a program to provide a solution to an arithmetic expression.

    • Syntax Defects

    A syntax defect occurs when there is a minor error in the syntax. When developers miss out on a simple semicolon (;) at the end of a statement, a curly bracket to delimit a code block or any other mistake in the syntax, it results in a syntax defect.

    • Logical Defects

    This type of defect results in customer dissatisfaction s as it causes when developers do not have a clear idea of the specified requirements. They think about the solution in a completely different way.

    • Performance Defects

    Performance defects occur when a system or software program fails to deliver the desired outcomes. It comprises the application's performance while being used with various loads.

    • Multithreading Defects

    This kind of defect occurs when you try to perform multiple tasks at the same time. When the deadlock and starvation (high priority processes get executed, leaving low priority processes behind for indefinite time) between different tasks take place, it results in an application’s failure.

    • Interface Defects

    When users try to interact with an application and spot defects, they are interface defects. With this type of defect, users cannot leverage the application painlessly.

    What is an Error?

    An error is generally a blunder, misunderstanding, or misconception on the side of an application developer. Sometimes, developers may write wrong spellings, misunderstand some sign notations, or do not have clear requirements; they make mistakes in the program code which we refer to as an error.

    There are several reasons for an error to occur, such as wrong logic or incorrect syntax. However, it results in several issues, including design issues, coding issues, and system specification issues.

    When there is any small mistake in the source code, which is an error, the actual results and the expected results definitely vary. This, in turn, makes the software application behave differently, leading to customer dissatisfaction.

    Here are some typical kinds of errors encountered during the development of software applications:

    • Communication Error: It takes place when an application is not able to communicate with end users. For instance, your application has no menu, no help instructions, etc.
    • Grammatical Error: When developers write wrong spellings and incorrect sentences, it gives rise to grammatical errors.
    • Missing Command Error: When developers miss out on any command necessary to include in the source code, this type of error occurs.
    • Calculation Error: It is an error that occurs due to bad logic, the use of incorrect formulae, data type mismatch, functional call issues, etc.

    What is a Fault?

    A fault in software development is any incorrect and unintended behavior of an application due to various aspects, such as lack of resources, not following proper instructions, etc. It generates warnings in the program. This implies that developers have not created the logic for handling errors in an application.

    If the fault is left unaddressed, it may lead to failure in the deployed source code. Even a single, minor fault can result in an error that may require a hefty amount to resolve. Adopting development methodologies, programming techniques, code analysis, and peer review can prevent faults in software products.

    The following are some different types of faults encountered in software testing:

    • Algorithm Fault: When the algorithm is not able to provide the correct and clear results for the given input, the algorithm fault occurs. It takes place due to incorrect processing steps, which can be prevented by disk checking.
    • Syntax Fault: When developers write the wrong syntax, it gives rise to syntax faults. Even a single syntax error leads to zero output.
    • Computational Fault: When the disk implementation goes wrong or if the program produces incorrect results, computational faults occur. For instance, when you combine floating point values with integer variables, it produces an unexpected result that leads to a computational fault.
    • Documentation Fault: When the developed source code does not comply with the documentation, this type of fault takes place.
    • Hardware Fault: This type of fault occurs when the specified hardware does not work properly with the desired software.
    • Software Fault: When the specified software is not compatible with the operating system, it results in a software fault.
    • Overload Fault: We know there are different types of data structures, like arrays, lists, etc., that serve as a memory for programs. An overload fault occurs when the memory is filled and utilized beyond its capacity.
    • Commission Fault: When developers make mistakes in the expression statement, a commission fault takes place. For example, when you initialize an integer value with a floating point value, it gives a commission fault.

    What is a Failure?

    A failure is a situation that occurs with the accumulation of several defects and results in the loss of data, making the software product unresponsive. Even a single defect can lead to one failure or several failures.

    For example, consider a bank application. If the Amount Transfer module of an application does not work for the end-users, it is a failure.

    Not only defects but also many other reasons, such as environmental conditions, can result in the failure of an application. Many times, human errors are responsible for the failure of applications when they provide incorrect input values. Moreover, not every defect leads to a failure; for instance, the defect in the dead code.

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

    We have discussed each term in detail above. In general, all these terms are used as synonyms. However, in software testing, each of these has a different meaning.

    • An error is a mistake done by developers.
    • A defect is an error that is found during SDLC.
    • A bug is a defect that is found during STLC (Software Testing Life Cycle).
    • When multiple defects leave an application unresponsive, it is a failure.
    • A fault is a situation that causes the software product to fail.

    Let us now have a look at the detailed differences between a bug, defect, error, fault, and failure.








    A bug is a defect, which indicates that an application is not functioning as intended.

    A defect is referred to as discrepancy in the actual and expected outcomes.

    When developers make mistakes while writing the source code, it results in an error because it leads to compilation and execution failure.

    A fault is a situation that makes a software product to fail by not allowing it to achieve its intended purpose.

    The combination of several defects leads to the failure of a software product and makes it unresponsive.


    Different types of bugs include:

    - Logical Bugs

    - Algorithmic bugs

    - Resource bugs

    Based on priority:

    - High

    - Medium

    - Low

    Based on severity:

    - Critical

    - Major

    - Minor

    - Trivial

    The following are the different types of errors:

    - Syntactic Error

    - User interface error

    - Flow control error

    - Error handling error

    - Calculation error

    - Hardware error

    - Testing Error

    Here are some major types of faults:

    - Business Logic Faults

    - Functional and Logical Faults

    - Faulty GUI

    - Performance Faults

    - Security Faults

    - Software/ hardware fault

    No types

    Raised By

    Test engineers identity bugs.

    A defect is raised by test engineers and resolved by developers.

    Developers and automation test engineers raise errors.

    End users find faults.

    Manual test engineers are in charge of identifying the failure in an application.


    - Missing coding

    - Wrong coding

    - Extra coding

    - Receiving and giving inaccurate input

    - A coding or logical error that causes software to malfunction

    - Incorrect code.

    - Impotence to properly build or run a software

    - Logical ambiguity in code

    - Uncertainty regarding requirements

    - Poor design and architecture

    - Logical error

    - Incorrectly designed data definition processes.

    - Software malfunctions are caused by a logical anomaly or by holes in the code.

    - System errors

    - Human errors

    - Environmental variables

    How to Prevent

    - Putting test-driven development into practice.

    - Modifying improved development procedures and assessing the code's cleanliness.

    - Putting innovative programming techniques into practice.

    - Leveraging correct and proper software coding practices .

    - Perform peer reviews and code reviews.

    - Validating bug patches and improving the software's general quality are both necessary.

    - Verify that the process has been retested.

    - Go through the specifications.

    - Group the problems.

    - Assess the mistakes.

    - Peer evaluation of test documents and requirements.

    - Checking the source code and design of software for accuracy.


    This was all about the difference between a bug, defect, error, fault, and failure in the software development life cycle. Each term has a different meaning, impacts different parts of an application, and has different causes. However, all these issues are linked and dependent on each other.

    All of them together slow down the performance of the software application. Hence, it becomes necessary to take preventive measures immediately.

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    While an error is an issue when developers make mistakes in writing the source code and lead to compilation or execution failure, a bug is an unexpected or unintended behavior of an application. A bug could be permanent, whereas an error is temporary.

    No, failure is the result of several defects that leaves the software application unresponsive, whereas an error is simply a coding error made by developers while writing the code.

    STLC stands for Software Testing Life Cycle. It is a structured approach to testing or validating software products in several phases to ensure they meet the quality standards and operate as expected.

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