It is based on the legal maxim “Ignorantia facti excusat, ignorantia juris non excusat” which means ignorance of facts excuses, ignorance of the law does not excuse. It has very limited use and is not readily accepted by the courts.
R v, Tolson[(1889) 23 QBD 168] – The appellant’s husband got lost at sea after one year of her marriage. Later, believing her husband to be dead, she married another man. Appellant’s husband turned up after some time she had remarried. Appellant was then charged with the offence of bigamy. Appellant’s defence of mistake was accepted it was reasonable in the circumstances to assume that her husband was dead.
Ranson v. Kitner, [31 III. App. 241 (1888)] – Defendant shot plaintiff’s dog, reasonably believing it to be a wolf. It was held that defendant is liable and plea of mistake could only be accepted if the plaintiff has wrongfully induced the mistake.