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Mistake Of Fact
Mistake of fact may sometimes be used as a defense to lessen or eliminate the element of mens rea in a crime. Unlike mistake of law, in which the defendant does not realize she is committing a crime because she's unaware of the law that criminalizes the action, a mistake of fact involves genuine ignorance of one or more material factors involved in the crime itself. In cases of liability without fault & strict liability, where mens rea is not a factor, a mistake of fact cannot generally be used in trial; however, in conventional cases it can be a very helpful defense.
For example, if Mr. Scott intends to purchase several thousand dollars worth of merchandise from a large hardware store, but the cashier fails to ring up a thousand dollars of this, he may be arrested in the parking lot by the store's security guard and charged with grand larceny, a serious felony. If Mr. Scott was genuinely unaware that some of his merchandise was unpaid for, and truly believed that he left the store as the legal owner of the merchandise, his lawyer might successfully plead that an honest mistake of fact eliminates mens rea from Mr. Scott's actions, thus rendering him innocent of grand larceny. (On the other hand, if the prosecution was skilled enough to demonstrate that Mr. Scott was in fact aware that he hadn't paid for the merchandise and intended to leave the store with unpaid merchandise, the mistake of fact defense would not hold.)
In situations when a mistake of fact cannot eliminate mens rea, it can sometimes reduce it. In the inappropriate use of deadly force, for example, if the defendant takes a life while honestly and reasonably - and mistakenly - believing that his own life was in danger, then a mistake of fact could be argued in court to reduce the charge from murder to a lesser homicide such as manslaughter. If the defendant is shown not to have acted negligently or unreasonably in the killing, the court could choose not to charge him with homicide at all, though this depends largely on the court. For something as serious as wrongfully taking a life, the defendant probably will not get off scott free; but displaying an honest and reasonable mistake of fact could mean the difference between a life outside or inside of prison.