A software bug is a coding error that causes an unexpected defect in a computer program. In other words, if a program does not perform as intended, it is most likely because of 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.
The top reasons for software bugs:
Lack of communication
Lack of organized communication leads to miscommunication. Most often this is the primary contributing factor for software bugs post development. Thereby, lack of clarity in communication can lead to an ambiguity in what the software should or shouldn’t do. In many occasions, the customer may not be completely clear on how the product should ultimately function. This is especially true when the software is developed for a completely new product. Such cases usually lead to a lot of misinterpretations from both sides.
Recurring ambiguity in requirements
Constantly changing software requirements cause a lot of confusion and pressure both on the development and testing teams. Often, adding a new feature or removing existing feature can be linked to the other modules or components in the software. Overlooking such issues causes bugs.
Missing process framework
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.
Too much rework
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. Furthermore, serious bugs can especially occur with larger projects, as it gets tougher to identify the problem area.
Self imposed pressures
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.