Exceptions are of two types viz excusable and justifiable. Mistake is an excusable Exception. It is contained in Chapter IV of IPC in Sections 76 and 79.
Section 76 of the IPC says that 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. It is clear from the section itself that only mistake of fact can be put forward as an Exception. 'Ignorantia juris non excusat' or ignorance of law is not an excuse according to the Section. So inorder to constitute an exception the necessary ingredients are as follows:
- There must be an act done
- The act must contain ingredients of an offence
- The act done must be due to ignorance of fact
- There must be good faith in doing the act ie there must be due care and caution or such belief
- Belief that he is bound by law to do the act.
A similar provision is laid down under Section 79 which says that nothing is an offence which is done by any person who is justified by law, 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 justified by law, in doing it. Thus if a person sees another commit an act which appears to the first person a murder and the first person in good faith seizes the other to be brought before the police, he has not committed an offence of wrongful restraint or confinement or like those. The reason is that he was under a mistake of fact and in seizing the other he has done a lawful thing in good faith.