Joseph Forsyth brought a case before the Supreme Court in 1850, arguing that he was wrongfully convicted of violating an act of Congress. The act prohibited citizens from trading with Native Americans without permission from the government. Forsyth argued that his conviction should be overturned because he had obtained permission to trade from two local Indian tribes and thus did not violate any laws. The Supreme Court ultimately ruled against him, finding that while it was true that Forsyth had received permission to trade with the local tribes, this did not exempt him from federal law which required all traders to obtain approval directly from Congress or other authorized agents of the United States government.

In the case of Joseph Forsyth v. The United States, the Supreme Court was tasked with determining whether a contract between two parties that had been made in violation of an existing federal law could be enforced by either party. Justice McLean wrote a dissenting opinion arguing that although it is true that contracts made in violation of federal laws are not enforceable, this particular contract should still be upheld because both parties were acting under mutual mistake and neither knew about the existence of the law at issue. He argued further that if one party was aware and chose to ignore it while entering into the agreement then they would bear all responsibility for any damages incurred as a result. Ultimately, he concluded that since both parties acted without knowledge or intent to violate any laws, their agreement should stand despite its illegality according to federal statutes.