Mistake of law is a type of criminal defense that is quite rare in modern law. In most cases, ignorance of the law is not an adequate defense, unless the defendant is deemed to be too young or mentally unable to have a reasonable understanding of laws and their consequences. A mistake of law defense suggests that a defendant, while having knowledge of the law, drew an erroneous conclusion about the legal status of an act.
One situation where a mistake of law defense might be applicable is if a judge, law officer, or other official of the court, explains a law to a person incorrectly. The theory behind this exception is that officers of the court and legal system are meant to be reliable experts, therefore their interpretation is reasonably expected to be sound. This exception does not apply to interpretations of law given by attorneys.
Another occasion in which a mistake of law may be a defense is if the defendant was following laws that were existent when the action was committed, but have since been overturned or changed. If, for instance, a person has his car towed for parking in an area that has been recently deemed a no-parking zone, but the area has not yet been marked or identified, nor has the decision been publicized in any way, he or she may be able to say that the towing was a result of a mistake of law.
The publication of laws is a very important consideration when discussing a mistake of law defense. In most common law jurisdictions, the legal body has a responsibility to make laws public and publicly available for review. If the public has no way to access the terms and conditions of a law, they cannot reasonably be expected to know or follow the law.
Judicial precedent has also generally supported the idea that clerical errors in court-originating documents can allow for a defense of mistake of law. If, for instance, if a written judgment reads that a convicted person must attend rehabilitation meetings twice a year, but was supposed to read twice a month, the defendant might be able to avoid violation charges. The argument, in this case, would be that the document was a verified court-issued judgment and, therefore, reasonably assumed to be correct.
A mistake of law is distinct from a mistake of fact. In a mistake of fact defense, the defendant suggests that an error was made through incorrect factual assumptions. For instance, if a person picked up the wrong to-go restaurant order, believing it to be his, he has committed a mistake of fact. If, however, he takes the to-go order because he believes he is legally entitled to it as the person who ordered it owes him money in the exact amount of the cost of the food, he may be making a mistake of law.