(CONTRACT LAW) an incorrect belief, at the time of contracting, that certain facts are true. It may be used as grounds to nullify (=make legally null and void; invalidate) the agreement.
A contract may be void if there is a mistake in the identity of the contracting party.
an error or unintentional act
An NHS (=UK National Health Service) trust has been ordered to apologise personally to relatives of a man who bled to death after a mistake during surgery.