Also known as: honest and reasonable mistakes · honest mistake · reasonable mistake
Written by attorneys · grounded in primary & secondary sources — see below
An excuse or privilege that arises when a person holds a subjectively genuine belief about a fact that, if true, would render the conduct lawful and that belief is also objectively reasonable under the circumstances. The doctrine operates by negating fault or the required mental state for the conduct in question. It applies only when the mistake concerns a material element and is not the product of the person's own fault.
Officer Ramirez attempted to arrest Hyun Han for suspected shoplifting. Han believed Ramirez was an imposter because of a prior false report and resisted with force. The resistance caused no injury. Because Han's belief that the arrest was unlawful rested on a reasonable mistake of fact, the privilege to defend against the officer's force was not lost.
Holly Hayes missed a filing deadline in a personal-injury suit. She paid the client from personal funds and never disclosed the potential malpractice claim or advised independent counsel. The honest mistake about the deadline itself did not trigger discipline, but the subsequent concealment and conflict created serious professional misconduct.
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Hana Huang purchased a used tractor from a dealer who had obtained it through a paperwork error. Huang believed the dealer held clear title. When the true owner reclaimed the tractor, the court reduced damages because Huang had converted the equipment in good faith under a reasonable mistake.
Hector Huerta met a young woman at a bar who produced what appeared to be valid identification showing she was eighteen. He took her to a hotel with her consent. Huerta's honest and reasonable belief about her age was raised as a potential defense to liability for the offense that would otherwise have been committed.
Officers executed a search warrant that later proved defective because the issuing magistrate had made an error in the supporting affidavit. The officers had no reason to doubt the warrant's validity. Evidence obtained during the search was admissible under the good-faith exception because the officers acted on an objectively reasonable mistake.
Hunter Hughes stood nearby when a fight broke out between two other men. He had arrived after the confrontation began and took no part in it. Because the evidence showed no prior arrangement and Hughes acted under an honest belief that he was merely an observer, instructions treating mere presence as sufficient for liability were erroneous.
The defense applies when the defendant's genuine belief, if true, would make the conduct lawful and the belief is objectively reasonable. It negates the required mental state for a material element of the offense. The mistake must not be caused by the defendant's own fault.
In many jurisdictions an honest belief suffices if it negates mens rea, but for certain offenses such as rape the mistake must also be reasonable. Reasonableness is required when the crime is defined in terms of negligence or when the jurisdiction follows the traditional rule for that offense.
A mistake of fact concerns the factual circumstances surrounding the act and can negate mens rea. A mistake of law generally does not excuse liability unless it is a reasonable reliance on an official misstatement of law or falls within the good-faith exception to the exclusionary rule.
…Amendment requirements. After today's decisions, however, that institutional incentive will be lost. Indeed, the Court's "reasonable mistake" exception to the exclusionary rule will tend to put a premium on police ignorance of the law. Armed with the assurance provided by today's decisions that evidence will always be admissible…