Fatigued after an 11-hour flight, Anna grabbed her well-worn suitcase off the luggage carousel. That bag had seen a lot of miles with her. She took a taxi home, left the suitcase near the front door, and collapsed into bed. She’d missed that bed!
Jet lag is rough. She woke up 12 hours later, still feeling bleary. After pouring coffee down her throat, she dragged her suitcase into the bedroom to unpack. But when she tried her usual combination, the lock wouldn’t open. She tried again. Nothing. Swearing, she thought to check the name tag.
Oh no. It was someone else’s battered black suitcase. Hers was probably still sitting at the airport, but that wasn’t her biggest worry. She’d stolen someone’s stuff! What if the police were already on their way?
Anna can relax. She’s not going to be making a trip to the Big House, at least not over the suitcase.
Mistake can sometimes be used as a defense to criminal charges—but only sometimes, because there are two kinds of mistake.
The first kind is called mistake of fact. It occurs when the defendant was wrong about some material fact, such as Anna believing she’d grabbed her own suitcase. As long as Anna’s mistake was reasonable, it will usually be successful as a defense.
Why? (This paragraph is law geekiness, and you are welcome to skip it if so inclined.) Crimes require that the defendant commit a particular act—or fail to do a particular act, like paying taxes owed. This is called the actus reus, the evil act. But most crimes also require that the defendant have a particular state of mind at the time of the action. This is called the mens rea, the evil mind. Larceny, for example, requires that the defendant take someone else’s property (that’s the actus reus) and that she do so intentionally (the mens rea). So if Anna honestly believes she’s grabbed her own suitcase, she didn’t intentionally take someone else’s stuff and therefore didn’t have the mens rea. So she can’t be convicted.
(Okay, extreme geekiness over.)
The other kind of mistake is mistake of law. This happens when a person believes that the law permits a particular action—but the person is wrong. So if I honestly believe it’s legal to light up a joint in Boise (it’s not), the cops can arrest me and I won’t be able to use mistake as a defense.
The reason behind this rule makes policy sense. If we allowed people to claim mistake of law, well, wouldn’t everyone just purposely avoid learning what the law says?
So here’s a real-life example. Every fall I host a barbecue at my house for my department colleagues. It’s the one (and generally only) time everyone comes to my place. I live in a subdivision with a lot of identical-looking houses. A couple of years ago, one of my colleagues drove to my house, opened the front door (unlocked), and marched inside—only to discover it wasn’t my house. Luckily for her sake, nobody there noticed and she crept back out. But if she had been caught, she’d have had a valid defense.