The U.S. Supreme Court this week heard oral arguments in two cases, United States v. Black and United States v. Weyhrauch, which offer the court a chance to limit a dangerous legal development. [IMGCAP(1)] Traditionally, only those who freely choose to commit wrongful acts should be punished as criminals:�All are entitled to be informed as to what the state commands or forbids,� the court announced in 1939. But in recent decades, Congress passed thousands of vague and sweeping new criminal laws, and the court has done little to limit their reach. As a result, ordinary Americans are now criminally charged for inadvertently violating hopelessly vague statutes or for making the kinds of reasonable mistakes that we all sometimes make in our professional lives.The Supreme Court�s decision to hear Black and Weyhrauch is a hopeful sign that help may be on the way. Both cases concern the scope of the federal mail and wire fraud laws. Unlike traditional state fraud laws, these federal statutes criminalize deceitful conduct even if no one is actually deceived, even if no harm is actually caused, and even if the perpetrator received no benefit. These laws carry draconian penalties of up to 20 years in federal prison, and they apply broadly to any �scheme or artifice to defraud� that involves, even indirectly, use of the U.S. Postal Service, e-mail or telephone.Congress amended these already-broad statutes in 1988 to extend their reach to �the intangible right to honest services� owed, for example, by an employee to a company or by an elected official to a state government. The �honest services fraud� amendment has led federal courts to effectively rewrite � on the fly � the rules of engagement for almost all fiduciary relationships. The cost of this approach is that no one knows, anymore, just what those rules are.Black is a very technical case involving the adequacy of certain jury instructions, but Weyhrauch illustrates this danger well. Bruce Weyhrauch is a former Alaska state legislator who is in prison for federal mail fraud pending the outcome of his Supreme Court appeal. In 2006, Weyhrauch mailed his résumé to an oil and gas company called VECO Corp. in order to be considered for a job when his term in office was over. Weyhrauch was convicted of mail fraud for afterward voting on bills that affected VECO�s interests without disclosing his potential conflict of interest as an applicant for a future job with the company.But the lengthy Alaska rules that described the ethical duties of state legislators did not require Weyhrauch to disclose such potential conflicts. Disclosure rules are prophylactic rather than substantive in nature, and Alaska�s omission of this duty in the context of rules that required disclosure in other situations suggests that the state made a deliberate policy choice not to require disclosure in these circumstances. Weyhrauch was not accused of actually selling favors, but only of violating a pro forma disclosure requirement that Alaska had specifically declined to impose.The trial judge initially concluded that Weyhrauch could not have committed honest services fraud because he had done everything that the state of Alaska asked of him. The 9th Circuit reversed that decision, however, stating that the vague federal law protecting the �intangible right to honest services� imposed additional, undefined duties on state lawmakers � and by extension, private employees � over and above the expectations of those to whom the �honest services� are actually owed.The honest services provision is so vague that it mocks the principle that citizens should know in advance what the law is. Unless the Supreme Court reverses Weyhrauch�s conviction, neither lawmakers nor employees can assure themselves that they are providing �honest services� simply by scrupulously meeting all job requirements. Instead, they must guess what additional requirements federal prosecutors and courts may impose after the fact.Nor are vague offenses like mail and wire fraud the only federal crimes that ordinary Americans inadvertently commit. Traditionally, a reasonable mistake about the facts is a defense to a criminal charge, but many modern federal criminal laws lack any language limiting their reach to those who commit harmful acts with mens rea, or a �guilty mind.� As a result, unlucky individuals can be convicted under federal law � and even sentenced to prison � for reasonable mistakes.Moreover, federal criminal laws increasingly punish those who violate the Code of Federal Regulations, a vast web of ever-changing regulatory requirements promulgated by bureaucrats. Even experts often disagree about what these regulations mean. But following the advice of such an expert in good faith is no defense for those unlucky enough to be criminally charged for misunderstanding the code.The Supreme Court has tolerated these and other excesses in the federal criminal law for too long, and the Black and Weyhrauch cases offer it a good opportunity to begin the long process of restoring important traditional limits on the reach of the criminal law.Marie Gryphon is a senior fellow at the Manhattan Institute�s Center for Legal Policy. Her new paper, �It�s a Crime? Flaws in Federal Statutes that Punish Standard Business Practice,� was released Dec. 9.