Failure: Any deviation of the observed behavior from the specified behavior. Error: The system is in a state such that further processing by the system will can lead to a failure. Fault: The mechanical or algorithmic cause of an error.

Fault: Fault could as simple as a wrong value.Fault have many causes including: 1. Misunderstanding of requirements 2. Overlooking special cases 3. Using the wrong variable 4. Typing mistake Failure: The inability of a system to perform its required functions within specified performance requirements.