The paper examines the characteristics of a mistake of the commitment of crime as an optional feature of the mental state of the crime. The analysis conducted offers an opportunity to state that in international criminal law, a mistake of law, although taken into account, does not generally affect the classification of crime. We uncovered and substantiated the need to research mistake as one of the features of the mental state of crime. This will make it possible to reveal the real causes of crime and administer the right type and measure of punishment. Moreover, the research provides a definition of mistake of criminal law by which we should understand an incorrect view of a person of the actual substance of his/her action and the specific characteristic of social relations that are protected by criminal law and are damaged as a result of a crime. Proceeding from a scientific approach to the definition of mistake of criminal law and its role in the mental state of corpus delicti, the author proposes that mistakes of law and mistakes of fact in criminal law affect the classification of crimes.