Error, Fault and Failure

Error- 

A person can make an error (mistake), which can lead to the introduction of a defect (fault or bug) in the software code or in some other related work product.

Fault-

 An error that leads to the introduction of a defect in one work product can trigger an error that leads to the introduction of a defect in a related work product. 

For example, a requirements elicitation error can lead to a requirements defect, which then results in a programming error that leads to a defect in the code. 

Failure:

If a defect in the code is executed, this may cause a failure, but not necessarily in all circumstances. 

For example, some defects require very specific inputs or preconditions to trigger a failure, which may occur rarely or never.

In addition to failures caused due to defects in the code, failures can also be caused by environmental conditions. 

For example, radiation, electromagnetic fields, and pollution can cause defects in firmware or influence the execution of software by changing hardware conditions