To be considered a crime worthy of punishment, a criminal act must generally have two components in place:  the physical act itself (actus reus) and the guilty intent behind that act (mens rea).  From the prosecutor's point of view, they must prove the existence of both elements of the act beyond a reasonable doubt in order to gain the conviction.  That task is made more difficult if the defendant is able to show that he was mistaken at the time he committed the criminal acts -  that he understood the facts, or, in some cases, even the law, to be different than they actually were.

So is it a valid excuse to claim you've made a mistake?  It depends.

There are basically two different kinds of errors or mistakes that a person can make, that might legally excuse them from culpability on criminal charges.   “Mistake of fact” is the first.  It involves an accused who holds an honest-but-mistaken belief in a set of facts that, if true, would otherwise have entitled him or her to be acquitted.   Essentially, mistake of fact excuses the accused from criminal liability because the requisite guilty state of mind (or mens rea) is missing.

Here’s a simple example:   A man buys a stereo system that – unbeknownst to him – is stolen.   His lack of knowledge will negate the criminal intent necessary to convict him of receiving stolen goods, which is a crime.

On the other hand, a “mistake of law” is of a different character; it involves a person who has full knowledge of the facts, but who comes to a mistaken conclusion as to their legal effect.    In other words, the accused fully appreciates what he or she is doing, but simply doesn’t appreciate the illegality of it.

For example, let’s say that it’s the middle of the night, and a driver pulls up to a deserted intersection with a stop sign.   She believes that, under such circumstances, there is no requirement to come to a full stop, and proceeds through the intersection without doing so.  This is a mistake of law – she knows what she is doing, but is working under the misapprehension that she is not committing a crime.

Unfortunately, in the criminal realm, both mistake of fact and mistake of law are only rarely successful as defences to a criminal charge.  Mistake of law in particular is difficult to establish, although naturally, there are some exceptions including those situations where:

1) The law prohibiting your conduct has not been published;
2) You have relied on a statute that was later overturned or held to be unconstitutional;
3) You relied on a decision of a court; and
4) You relied on an interpretation by an appropriate official (also known as “officially-induced error”).

Still, these two categories of mistake are simply variations on two long-established legal principles:  Mistake of fact is part-and-parcel of the concept that in order to be criminally convicted, an accused must have a guilty mind.   Mistake of law, on the other hand, is just a variation of the public policy reflected in the legal saying, ignorantia legis neminem excusat (or “ignorance of the law is no excuse”).

So the bottom line is that everyone – with the exception of the criminally insane – is presumed to know the law.  In that respect, there’s no excuse…. or at least not one you’d want to rely on.

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The preceding article should not be taken, or relied upon, as legal advice.  It is presented "as is," for general information purposes and commentary.