The terms: error, defect, bug, and failure are closely related in software development. In a nutshell: An error leads to a defect, and when these defects go undetected, they lead to failure.


An error is a mistake made by a developer in the code. Its main causes are:

  • Negligence: Carelessness often leads to errors
  • Miscommunication: An unclear feature specification or its improper interpretation by developers could lead to slips.
  • Inexperience: Inexperienced developers often miss out on essential details, leading to faults down the line.
  • Complexity: Intricate algorithms can cause developers to make mistakes in their coding logic.
Defect & Bug

A system is said to have a defect is when its expected and actual behavior differ. A defect, when found by a tester or end-user, is called a bug. Its main causes are:

  • Missing test cases
  • Test cases not executed by the tester
  • Improper execution of test cases
  • Code changed after testing
When one or more bugs cause a significant loss of functionality, a failure is said to have occurred.
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