Nothing is an offence which is done by a person who is, or who by reason of a mistake of fact and not by reason of a mistake of law in good faith believes himself to be, bound by law to do it.
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(a) A, a soldier, fires on a mob by the order of his superior officer, in conformity with the commands of the law. A has committed no offence. (b) A, an officer of a Court of Justice, being ordered by that Court to arrest Y, and after due enquiry, believing Z to be Y, arrests Z. A has committed no offence.