What is a Bug? Why do Bugs occur?

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A software bug may be defined as a coding error that causes an unexpected defect, fault, flaw, or imperfection in a computer program. In other words, if a program does not perform as intended, it is most likely a bug.

There are bugs in software due to unclear or constantly changing requirements, software complexity, programming errors, timelines, errors in bug tracking, communication gap, documentation errors, deviation from standards etc.

  • Unclear software requirements are due to miscommunication as to what the software should or shouldn’t do. In many occasions, the customer may not be completely clear as to how the product should ultimately function. This is especially true when the software is a developed for a completely new product. Such cases usually lead to a lot of misinterpretations from any or both sides.
  • Constantly changing software requirements cause a lot of confusion and pressure both on the development and testing teams. Often, a new feature added or existing feature removed can be linked to the other software modules or components in the software. Overlooking such issues causes bugs.
  • Also, fixing a bug in one part/component of the software might arise another in a different or same component. Lack of foresight in anticipating such issues can cause serious problems and increase in bug count. This is one of the major issues because of which bugs occur since developers are very often subject to pressure related to timelines; frequently changing requirements, increase in the number of bugs etc.
  • Designing and re-designing, UI interfaces, integration of modules, database management all these add to the complexity of the software and the system as a whole.
  • Fundamental problems with software design and architecture can cause problems in programming. Developed software is prone to error as programmers can make mistakes too. As a tester you can check for, data reference/declaration errors, control flow errors, parameter errors, input/output errors etc.
  • Rescheduling of resources, re-doing or discarding already completed work, changes in hardware/software requirements can affect the software too. Assigning a new developer to the project in midway can cause bugs. This is possible if proper coding standards have not been followed, improper code documentation, ineffective knowledge transfer etc. Discarding a portion of the existing code might just leave its trail behind in other parts of the software; overlooking or not eliminating such code can cause bugs. Serious bugs can especially occur with larger projects, as it gets tougher to identify the problem area.
  • Programmers usually tend to rush as the deadline approaches closer. This is the time when most of the bugs occur. It is possible that you will be able to spot bugs of all types and severity.
  • Complexity in keeping track of all the bugs can again cause bugs by itself. This gets harder when a bug has a very complex life cycle i.e. when the number of times it has been closed, reopened, not accepted, ignored etc goes on increasing.


  1. I know that bugs and defects are an integral part of the software testing process. Most of us are aware of this fact but many of us don’t know the real reason why their is bug in software in the first place and found out this information which is very important for students like us. Thank You.

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