Why Does Software Have Bugs?

What is a software bug?
A software bug is a failure or flaw in a program that produces undesired or incorrect results. It’s an error that prevents the application from functioning as it should.
Why does Software have bugs?
There are many reasons for software bugs. Most common reason is human mistakes in software design and coding.
Once you know the causes for software defects it will be easier for you to take corrective actions to minimize these defects.

Top 20 reasons for software bugs

1. Miscommunication or no communication
Success of any software application depends on communication between stakeholders, development and testing teams. Unclear requirements and misinterpretation of requirements are two major factors causing defects in software. Also defects are introduced in development stage if exact requirements are not communicated properly to development teams.
2. Software complexity
The complexity of current software applications can be difficult to comprehend for anyone without experience in modern-day software development. Windows-type interfaces, client-server and distributed applications, data communications, enormous relational databases, and sheer size of applications have all contributed to the exponential growth in software/system complexity. And the use of object-oriented techniques can complicate instead of simplify a project unless it is well-engineered.
3. Programming errors
Programmers, like anyone else, can make mistakes. Not all developers are domain experts. Inexperienced programmers or programmers without proper domain knowledge can introduce simple mistakes while coding. Lack of simple coding practices, unit testing, debugging are some of the common reasons most issues get introduced at development stage.
4. Changing requirements
The customer may not understand the effects of changes, or may understand and request them anyway – redesign, rescheduling of engineers, effects on other projects, work already completed that may have to be redone or thrown out, hardware requirements that may be affected, etc. If there are many minor changes or any major changes, known and unknown dependencies among parts of the project are likely to interact and cause problems, and the complexity of keeping track of changes may result in errors. Enthusiasm of engineering staff may be affected.
In some fast-changing business environments, continuously modified requirements may be a fact of life. In this case, management must understand the resulting risks, and QA and test engineers must adapt and plan for continuous extensive testing to keep the inevitable bugs from running out of control.
5. Time pressures
Scheduling of software projects is difficult at best, often requiring a lot of guesswork. When deadlines loom and the crunch comes, mistakes will be made. Unrealistic schedules though not common but major concern in small scale projects/companies results in software bugs. If there is not enough time for proper design, coding and testing, it’s quite obvious that defects will be introduced.
6. 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 have no idea if we can do that; we’ll wing it’
‘I can’t estimate how long it will take, until I take a close 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 problem’s’, the result is software bugs.

7. Poorly documented code
It’s tough to maintain and modify code that is badly written or poorly documented; the result is software bugs. In many organizations management provides no incentive for programmers to document their code or write clear, understandable code. In fact, it’s 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’).
Any new programmer starting to work on this code may get confused due to complexity of the project and poorly documented code. Many times it takes longer to make small changes in poorly documented code as there is huge learning curve before making any code change.
8. Software development tools
Visual tools, class libraries, compilers, scripting tools, etc. often introduce their own bugs or are poorly documented, resulting in added bugs. Continuously changing software tools used by software programmers. Keeping pace with the different versions and their compatibility is a major ongoing issue.
9. Obsolete automation scripts
Writing automation scripts takes lot of time especially for complex scenarios. If automation teams record/write any test script but forget to update it over the period of time that test could become obsolete. If the automation test is not validating the results properly it won’t be able to catch the defects.
10. Lack of skilled testers
Having skilled testers with domain knowledge is extremely important for success of any project. But appointing all experienced testers is not possible for all companies. Domain knowledge and the tester’s ability to find defects can produce high quality software. Compromise on any of this can result in buggy software.
Here are few more reasons for software bugs. These reasons are mostly applicable for software testing life cycle:  
11. Not having proper test setup (test environment) for testing all requirements
12. Starting to write code or test cases without understanding the requirements clearly.
13. Incorrect design which leads to issues being carried out in all phases of software development cycle.
14. Releasing software patches frequently without completing the software testing life cycle.
15. Not providing training to resources for the skills needed for developing or testing the application properly.
16. Giving very less or no time for regression testing.
17. Not automating repetitive test cases and depending on the testers for manual verification every time.
18. Not prioritizing test execution.
19. Not tracking the development and test execution progress continuously. Last minute changes are likely to introduce errors.
20. Wrong assumption made while coding and testing stages.

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