Error is an undersirable deviation from requirement. any
problem or cause for many problems which stops the system to
perform its functionality.

Failure - any expected action that is suppose to happen.
absence of expected response for any request.


bug - any missing functionality or any actin that is
performed by the system which is not supposed to be performed
" is an error found BEFORE the appln goes into production?

Reason for the birth of bug
Wrong functionality
missing functionality
extra or unwanted functionality