Ignorantia Facti Excusat, is a Latin Maxim which means “Ignorance of fact can be excused”. This maxim is the principle behind the concept of fact which is considered a valid and sufficient defense in criminal law. It means that any act done under a mistaken impression of a material fact is excused. In U.S vs Ah Chong (1910), it is explained that ignorance or mistake of fact works an acquittal if it is sufficient to negate a particular intent which under the law is a necessary ingredient of the offense charged. The mistake of fact must be honest and reasonable i.e., bona fide in nature, and must relate to the fact, not to law. A defendant cannot claim later that he was under the mistake of fact when he knew about the situation.
However, Section 76 of the Indian Penal Code, 1860, explains the provision about “mistake of fact”. The exemption on the ground of mistake of fact is based on the principle that a man who is mistaken or ignorant about the existence of a fact cannot establish an intention to constitute a crime, and hence, is not responsible in law for his deeds. Under S. 76 of IPC, 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. For instance, 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 offense.In, State of West Bengal vs. Shew Mangal Singh (1981 AIR 1917, 1982 SCR (1) 360), the accused police officers were on patrol when they were attacked by a mob. The Deputy Commissioner of Police ordered to open fire and a constable got injured. It was observed that the accused constables were bound by law to obey the orders of the superior officer. Both the HC and SC ruled that the situation was warranted and justified by the order to open fire and therefore, the accused got protection under section 76 and was held not guilty.