Can Mistake of Fact or Mistake of Law Be a Defense?

Learn when a defendant’s mistake can and can’t be a defense to a criminal charge.

By , Attorney · Mitchell Hamline School of Law
Updated March 20, 2024

Mistake of fact and mistake of law are legal defenses that a defendant might raise to dispute criminal charges. However, both pose challenges and their differences aren't always clear. Read on to learn more about these defenses.

What Is Mistake of Fact?

A mistake of fact refers to a person's misunderstanding as to the facts of a situation. The factual misunderstanding results in the person breaking the law.

Can Mistake of Fact Be a Defense to Criminal Charges?

Sometimes. If the person's mistake negates an element of a crime and the mistake was reasonable, a mistake of fact may be a defense.

Suppose John tells Marlene she can borrow his lawnmower but he accidently gives her the wrong address. Marlene heads to the address John gave her and sees a lawnmower in the front yard. She loads it up on her trailer just as the homeowner pulls up the driveway and calls the police. The police arrest her for theft.

In this situation, Marlene's reasonable mistake will likely clear her from theft charges. To commit theft, a person must take another's property with the intent of permanently depriving the owner of their property. Marlene did not take the lawnmower with the intent to steal it from the homeowner. She reasonably relied on the information John gave her. Without that specific intent, she didn't commit theft.

When Is Mistake of Fact Not a Defense?

In a few key instances, a mistake of fact will rarely be a defense.

Unreasonable mistake. Mistakes of fact generally won't work as a defense if the mistake was unreasonable. Let's say John invites Marlene over for an open house. He gives her the correct address and says to look for the yellow house. Marlene drives down the street, pulls up to the first yellow house she sees, and walks in. The alarm goes off and police arrive minutes later. They arrest her for trespassing. Marlene probably won't get out of these charges because her mistake wasn't reasonable. She had the correct address and didn't bother to look at it.

Strict liability crimes. Another instance where mistake of fact doesn't generally absolve a person from criminal liability involves strict liability crimes, such as statutory rape or speed limit laws. Strict liability crimes don't require that a defendant acted with any specific knowledge or intent. In other words, mistake of fact is irrelevant. For instance, a person commits statutory rape by having sexual intercourse with a child under the age of consent. It doesn't matter if the defendant believed the child to be older. Similarly, a person who drives over the speed limit without realizing it won't avoid a speeding ticket.

What Is Mistake of Law?

A mistake of law means a person misunderstood a law or didn't know it existed. Misunderstanding or ignorance of the law results in the person committing a crime.

Can Mistake of Law Be a Defense?

As a general rule, no. Mistake of law is rarely a defense. To allow a mistake of law to be a defense would be to encourage people to stay ignorant or claim ignorance of the law. It would invite people to justify wrongful acts based on their interpretation of what the law meant to them. In other words, the laws would lose their certainty and enforceability.

Example of Mistake of Law

Suppose Casey, a long-time resident of Colorado and regular pot smoker, moves to a new state. He brings his recreational weed with him, assuming recreational weed is legal in every state. Casey was wrong and, within a week, he was in jail on marijuana possession charges. Although Casey didn't intentionally break the law, he doesn't have a legitimate defense based on his mistaken belief that recreational marijuana use was legal in every state.

What if Casey had tried to look up the marijuana laws in his new state and misunderstood the legal difference between medical and recreational marijuana? Medical marijuana is legal (with a prescription) but recreational marijuana is not. Would this mistake of law be a defense? Again, no. Reliance on one's own interpretation of the law won't get you off the hook. Even if Casey had contacted an attorney who gave bad and incorrect advice, reliance on the attorney's opinion still wouldn't be a defense.

Is Mistake of Law Ever a Defense?

In very limited circumstances, mistake of law can be a defense. Let's say Casey's new state of residence made recreational weed legal, but the law was challenged. While the law goes through the courts, the state's attorney general incorrectly puts out an opinion stating that weed is still legal pending the court's decision. If Casey relied on this government interpretation, he might be able to raise a mistake of law defense if he's charged with illegal possession.

Talk to a Lawyer

As noted above, the differences between mistake of fact and law can be difficult to ascertain. To add to the confusion, every state has different laws and court interpretations of those laws. One state might make an exception to the general rules noted above. For instance, some states allow a mistake of fact defense in certain statutory rape cases (but not in all cases). If you face criminal charges, contact a criminal defense attorney to help you understand the laws and navigate the criminal legal system.

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