Rule 404(b) of the Federal Rules of Evidence has long served as a gatekeeping mechanism in criminal and civil trials, barring parties from using evidence of prior conduct to argue that a person acted in conformity with a character trait on a particular occasion. Although the rule is most often invoked to exclude evidence of prior bad acts, courts have also applied it to bar evidence of prior good conduct being offered to suggest that a defendant acted lawfully.

However, a recent decision from the U.S. Court of Appeals for the Second Circuit clarifies an important limitation of that principle. When prior conduct is offered not to show character, but to corroborate a defendant's account of events bearing on their state of mind, Rule 404(b) may not apply at all.

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