Generally speaking, there are bugs in software because of unclear requirements, software complexity, programming errors, changes in requirements, errors made in bug tracking, time pressure, poorly documented code and/or bugs in tools used in software development.
* There are unclear software requirements because there is miscommunication as what the software should or shouldn't do.
* Software complexity. All of the followings contribute to the exponential growth in software and system complexity.
* Programming errors occur because programmers and software engineers, like everyone else, can make mistakes.
* As to changing requirements, in some fast-changing business environments, continuously modified requirements are a fact of life. Sometimes customers do not understand the effects of changes, or understand them but request them anyway. And the changes require redesign of the software, rescheduling of resources and some of the work already completed have to be redone or discarded and hardware requirements can be effected, too.
* Bug tracking can result in errors because the complexity of keeping track of changes can result in errors, too.
* Time pressures can cause problems, because scheduling of software projects is not easy and it often requires a lot of guesswork and when deadlines loom and the crunch comes, mistakes will be made.
* Code documentation is tough to maintain and it is also tough to modify code that is poorly documented. The result is bugs. Sometimes there is no incentive for programmers and software engineers to document their code and write clearly documented, understandable code. Sometimes developers or programmers feel they cannot have job security if everyone can understand the code they write, or they believe if the code was hard to write, it should be hard to read.
* Software development tools, including visual tools, class libraries, compilers, scripting tools, can introduce their own bugs. Other times the tools are poorly documented, which can create additional bugs.