“Mistake” simply means a misguided judgment or act. In IPC, there are two instances of such a mistake. However, the mistake of fact only is excepted. There is no excuse for a mistake of law.
Mistake of fact in Section 76
If a person does an act by reason of a mistake of fact and in good faith believing himself to be bound by law to do such an act, it will not be counted as an offense. This is made clear by Section 76 of IPC. It is also made clear that the mistake of law is not enough to attract the exception. A, being an officer of Court under order from that Court to arrest B, arrests C after due inquiry and believing C to be B. Here A has committed no offense.
Similarly, if a police officer, upon the command of his superior, fires a mob and kills someone, he is not liable as he was under the compulsion of law/order from his superior officer. But the order must be a legal one or justified by law. One cannot go to the extent of killing someone merely because it was ordered so by his superior officer.
Section 79
Another instance of the exception of “mistake” is dealt with under Section 79 of IPC. A killed B in self-defense. This was witnessed by C and C thought that A intentionally murdered B. C, in good faith, seizes A and brings him before proper authorities in the exercise of the power which law gives him apprehending murders. Here, he has not committed any offense though it may turn out that A has committed the act in self-defense. So nothing will be counted as an offense that is done by any person who is justified by law or by reason of a mistake of fact and in good faith believes himself to be justified by law to do it. In the above illustration, C’s act is justified by law though he has been mistaken the fact that A acted in self-defense.
In Regina vs Prince[1], a man was brought to trial for taking away an unmarried girl of 16 years of age. Later, the jury found that the man was under a mistake of fact that the age of the girl is 18. The girl herself had told him that she is 18. Here the accused believed her words in good faith and takes her away. He is justified under the exception provided above.
See the factors required to plead the general exception of mistake here.
As per Section 105 of the Indian Evidence Act, the burden of proving the existence of circumstance falling within the exception above is upon the accused person.
Footnote
1. L.R. 2 C.C.R. 154 (1875)