Phases of introduction of bugs in software

A software bug is an error, flaw or fault in a program that makes it produce wrong or unexpected results. Bugs can be introduced in any phase of the software life cycle:

1. Requirements phase: missing, incomplete, ambiguous or wrong requirements; misunderstanding between customer and developers; later changes of the requirements. These are the most costly bugs because the whole product is built wrongly.
2. Design phase: wrong architecture, poor algorithms, faulty logic, ignoring timing or interface issues, poor module partitioning and wrong data structures.
3. Coding (implementation) phase: syntax and logic errors, wrong variable use, uninitialised variables, off-by-one errors, array overflow, memory leaks, pointer errors, wrong data types, inconsistent interfaces, and failure to follow the standards.
4. Testing phase: wrong or insufficient test cases, incorrect test environment and fixes that add new bugs (regression).
5. Integration and installation phase: interface mismatches between modules, wrong configuration, version conflicts and environment differences (hardware/OS).
6. Maintenance phase: modifications and patches that introduce new bugs or break working parts; poor documentation.

The cost of removing a bug rises sharply the later it is found, so reviews and testing at each phase are important.