This tutorial discusses the top 20 reasons “Why Does Software Have Bugs”. Understand why do bugs and failures occur in software:

What is a Software Bug?

A Software Bug is a failure, flaw, or error in a program that causes undesired or incorrect results or behaves in an unintended way. It is an anomaly (error/unexpected behavior) that prevents the application from functioning as it was expected to.

Why Does Software Have Bugs

Why does software have bugs

Why software has defects is a very broad question and at times can be purely technical. There are many reasons for the occurrence of Software Bugs. Some people that are not so tech-savvy call them computer bugs.

The most common reasons are human errors and mistakes made in designing the program and writing the source code. Another prominent reason could be incorrect interpretation while getting the software requirements.

Once you get to know why the software is having defects, and the causes for bugs, then it will be easier to take corrective actions to resolve and minimize these defects.

software bugs

Top 20 Reasons for Software Bugs

Let us understand in detail.

#1) Miscommunication or No Communication

The success of any software application depends on the organized communication between stakeholders, development, and testing teams, during various stages of the software development process. A lack of organized communication often leads to miscommunication.


Proper communication should start right from the time of requirement gathering, then its translation/interpretation to the document and continue during SDLC.

If the requirements remain unclear and incorrectly translated into specifications, the software is bound to have defects due to ambiguity in requirements. Certain Software Defects get introduced into the development stage itself if the developers are unaware of the right specifications.

Also, communication errors can happen if the software application is developed by some ‘X’ developer and maintained/modified by some other ‘Y’ developer.

Example: The development team deploys a new change but the testing team is unaware of it. Testers might suddenly find some unintended behavior and a lot of time can go into finding the cause. Proper communication within the team could have avoided this problem. So, effective communication remains the key to software development.

Also read =>

#2) Software Complexity

The challenging complexity of the current software applications can be difficult to adapt to for anyone with little experience in modern-day, almost daily changing software development methods and techniques.

The enormous rise of various third-party libraries, Windows-type interfaces, Client-Server, and Distributed Applications, Data Communication Systems, large relational databases as well as free RDBMS, varied techniques for building APIs, a large number of development IDEs, and the sheer size of applications have all contributed to the exponential growth in software/system complexity.

Unless the project/program is well designed, using object-oriented techniques can complicate the entire program, instead of simplifying it.

Example: Assuming, in a program there are too many nested if-else statements and unfortunately in user interaction one of the logical paths gets triggered which was unintentionally missed in testing although rigorous testing was done.

This might very well lead to a software bug and debugging & fixing it could be a real nightmare. This cyclomatic complexity can be reduced using switch cases or ternary operators, as applicable.

#3) Lack of Designing Experience/Faulty Design Logic

As the design is the very core of SDLC, quite a good amount of brainstorming and R&D is required to arrive at a reliable and scalable design solution.

But, many times self-imposed timeline pressures, lack of patience, improper knowledge of technical aspects, and a lack of understanding of technical feasibility can all lead to faulty design and architecture which in turn will introduce several software defects at various levels of SDLC, resulting in extra cost and time.

Example: The popular communication app ‘Slack’ had received criticism for its public DM feature. Although a useful feature, allowing users (friends) from outside the organization to participate in chat was unacceptable to many organizations. Perhaps the Slack development team could have given more thought while designing this feature.

#4) Coding/Programming Errors

Programmers, like anyone else, can make common programming mistakes and may use ineffective coding techniques. This may involve poor coding practices like no code review, no unit testing, no debugging, unhandled errors, faulty input validations, and missing exception handling.

Along with these, if the developers use the wrong tools, for instance, faulty compilers, validators, debuggers, performance checking tools, etc., then there is a very high probability that lots of bugs will creep up in the application.

Also, not all developers are domain experts. Inexperienced programmers or developers without proper domain knowledge can introduce simple mistakes while coding.

Example: Clicking on the ‘Cancel’ button does not close the window (which was expected behavior), although entered values are not saved. This is one of the simplest and most often found bugs.

#5) Ever-Changing Requirements


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Continuously changing requirements may be a reality and fact of life in some fast-changing business environments and market needs. The motivation and enthusiasm of the development team may be certainly affected, and the quality of work may be reduced significantly.

Various known and unknown dependencies need to be taken care of while working on many such minor or major changes. A significant amount of QA effort may be required and if not done properly may bring in many bugs in software. Keeping track of all such changes is again an overhead and complex task, which may further result in more application errors

In such cases, the management must understand and evaluate the resulting risks, and QA & test engineers must adapt and plan for continuous extensive testing to keep the inevitable bugs from running out of control. All these will require much more time than the originally estimated time effort.

Example: For an e-commerce website, the initial requirement was for product search to be based on product name, but a few days before the release, it was decided to have a search based on product code as well. In this case, improper testing or insufficient testing time will almost certainly result in bugs getting leaked into the production environment.

Also, read => Continuous Changing Requirements – How to keep up

#6) Time Pressures (Unrealistic Time Schedule)

As we all know, scheduling time and effort for a software project is a difficult and complex task, often requiring a lot of guesswork and historical data. When deadlines loom and the pressure mounts, mistakes will happen. There could be bugs in coding – some or many.

Unrealistic schedules, though not common, are a major concern in small-scale projects/companies resulting in software bugs.

As a result of unrealistic release schedules, and project deadlines (internal/external), software developers may have to compromise on certain coding practices (no proper analysis, no proper design, less unit testing, etc.), which can increase the probability of bugs in software.

If there is not enough time for proper testing, it’s quite obvious that defects will leak. Last-minute feature/design changes can also introduce bugs, at times most dangerous software bugs.


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Example: Drag and book a resource (event) functionality (which was a high priority was developed for a multi-storefront application under a tight deadline. However, due to limited testing time, there were DST issues.

#7) Egotistical or Overconfident People

People prefer to say things like:

  • ‘No problem’
  • ‘Piece of cake’
  • ‘I can whip that out in a few hours
  • ‘It should be easy to update that old code’

Instead of:

  • ‘That adds a lot of complexity and we could end up making a lot of mistakes’.
  • ‘We do not know if we can do that; we’ll wing it’.
  • ‘I can’t estimate how long it will take until I take a closer look at it’.
  • ‘We can’t figure out what that old spaghetti code did in the first place’.
  • If there are too many unrealistic ‘no problems’, then it results in software bugs.

Making assumptions by being overconfident will certainly bring in some or many software bugs at each stage of SDLC.

Example: One of the developers was quite sure of taking care of the performance aspect while downloading a large chunk of data in MS Excel and suggested the tester not to carry out more tests with such data. Eventually, it resulted in a bug in the production environment.

#8) Poorly Documented Code

It is tough to maintain and modify code that is badly written or poorly documented; the result is Software Bugs. In many organizations, management provides no real incentive for programmers to document their code or write clear, understandable code. Many times there are no set processes and standards for coding.

In fact, it is usually the opposite, they get points mostly for quickly turning out code, and there’s job security if nobody else can understand it (‘if it was hard to write, it should be hard to read’).

This poorly written and not so well documented code becomes a maintenance nightmare. Any new programmer working on this code may get confused because of the complexity of the project and the poorly documented code.

Many times it takes a longer time to make even slight changes in poorly documented code, as there is a huge learning curve before making any code change.

Example: The testing team discovered several bugs as a result of the migration of some screens of a legacy product (which has been in use for over 20 years) from C++ to C#. The main reason was a lack of proper code documentation and/or comments.

Also Read => Importance of Documentation

#9) Software Development Tools (Third-party Tools and Libraries)

Visual tools, class libraries, shared DLLs, plug-ins, npm libraries, compilers, HTML editors, scripting tools, etc. often introduce their own bugs or are poorly documented, resulting in added bugs.

Software engineers tend to use continuously and rapidly changing/upgrading software tools. Keeping pace with the different versions and their compatibility is a real and major ongoing issue.

Example: Defects in Visual Studio Code or deprecated Python libraries add their own level of disadvantages/challenges to writing effective software.

=> Read more on Software Development Tools

#10) Obsolete Automation Scripts or Over-Reliance on Automation

The initial time and effort taken to write automation scripts are quite high, especially for complex scenarios. If manual test cases are not in proper shape, then the time required will significantly increase.

Automation scripts need to be maintained regularly, wherever required, as per the changes done in the application. If the changes are not done on time then those automation scripts can become obsolete.

Also, if the automation test script is not validating the right expected outcome, then it will not be able to catch the defects and it does not make any sense to rely on these scripts.

Being excessively reliant on automation testing can cause manual testers to miss bug(s). For successful automation testing experienced and dedicated personnel is required. Also, the support of management is of utmost importance.

Example: After the product enhancement, one of the automation test scripts was not updated in time. Furthermore, bugs were discovered late in the testing cycle because the corresponding manual test cases were not executed due to the presence of the automated script. This added to the delay in software delivery.

#11) Lack of Skilled Testers

Having skilled testers with domain knowledge is extremely important for the success of any project. Domain knowledge and the tester’s ability to find defects can produce high-quality software. But appointing all experienced testers is hardly possible for all companies as the cost factor and team dynamics come into the picture.

Compromise on any of this can result in buggy software.

Poor and insufficient testing is becoming the new norm or standard in many software companies. Testing is being taken lightly which may involve a lack of proper or no test cases, flaws in the testing process, and the process itself getting done without giving much importance. All of these factors can certainly cause various types of software bugs.

Example: One good example could be insufficient DST-related testing for the event booking software feature.

#12) Absence or Inadequate Version Control Mechanism

The development team can easily keep track of all the changes done to a code base with the use of proper version control tools/mechanisms. Lots of software errors definitely will be observed without having any version control of the code base.

Even while using version control, the developer should take care to ensure that he/she is having the latest version of the code before committing any changes to the relevant code file.

Example: If the developer commits changes to more than one task at once (which is not standard practice), reverting the code to the previous version (which may be required if the latest commit causes build issues, etc.) will be extremely difficult. As a result, new bugs may be introduced during the development phase.

#13) Frequent Releases

Releasing software versions (for example, patches) frequently may not allow the QA to go through the complete regression test cycle. This is one of the major reasons nowadays for having bugs in the production environment.

Example: The PDF download feature of a multi-storefront application began breaking in the production environment because the tester neglected testing of this feature due to insufficient time and the fact that it was only checked in the previous release, and no changes were made to this feature.

#14) Insufficient Training for Staff

Even for experienced staff some training may be required. Without sufficient training on required skills developers can write incorrect logic and testers may design not-so-accurate test cases, resulting in software bugs and errors at various stages of the SDLC and testing life cycle.

This may also involve misinterpretation of the gathered requirements/specifications.

Example: A survey application was collecting data, which could be downloaded as an MS Excel file. However, due to a lack of technical knowledge, the developer failed to consider performance issues that could arise as a result of a large amount of data.

When the record count reached 5000, the application began to hang for hours with no result. This test was also missed by the tester, most likely due to insufficient training.

#15) Changes at the Eleventh Hour (Last-Minute Changes)

Any changes done at the last minute either in the code or any dependencies (e.g. hardware requirement, version of libraries used) can cause the most dangerous software bugs and failures.

Example: The version of a third-party library in one of the web applications was changed just two days before the release. The tester clearly did not have enough time to test, and there was defect leakage into the production environment.

#16) Ineffective Testing Life Cycle

  • Test cases are written without a proper understanding of requirements.
  • No proper test setup (test environment) for different environments.
  • Lack of traceability matrix
  • Insufficient time is given for regression testing
  • Lack of proper bug report
  • Incorrect or missing test execution prioritization
  • No importance is given to the testing process.

Here are a few more reasons for Software Bugs. These reasons mostly apply to Software Testing Life Cycle:

#17) Not Automating Repetitive Test Cases and depending on the testers for manual verification every time.

#18) Not tracking the development and test execution progress continuously.

#19) The incorrect design leads to issues being carried out in all phases of the Software Development Cycle.

#20) Any wrong assumption(s) made during coding and testing stages.


There are several reasons for the occurrence of software bugs. A list of the top 20 reasons was mentioned in this tutorial with a basic explanation. We hope you identified with a few or maybe many of the items we listed.

Please share your thoughts in the comments section below and do mention any other reasons you are aware of.

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