Pennywit v. Eaton was a United States Supreme Court case that dealt with the issue of whether a state court had the authority to issue a writ of error to a federal court. The case arose when the plaintiff, Pennywit, sued the defendant, Eaton, in a state court in Pennsylvania. The state court issued a writ of error to the federal court, which the defendant argued was an unconstitutional exercise of power. The Supreme Court held that the state court did not have the authority to issue a writ of error to a federal court. The Court reasoned that the state court was not a court of the United States, and therefore did not have the power to issue a writ of error to a federal court. The Court also noted that the writ of error was an extraordinary remedy, and that the state court should not be allowed to interfere with the proceedings of a federal court. The Court's decision in Pennywit v. Eaton established that state courts do not have the authority to issue a writ of error to a federal court. This decision has been cited in numerous subsequent cases, and has been used to support the principle that state courts should not interfere with the proceedings of federal courts.

In Pennywit v. Eaton, the United States Supreme Court was tasked with determining whether a contract between two parties could be enforced when it had been made without consideration and in violation of a state statute. The majority opinion held that the contract was unenforceable because it lacked consideration and violated public policy as expressed by the state law. Justice Field dissented from this decision, arguing that while contracts lacking consideration are generally void, there were exceptions to this rule which should have applied in this case. He argued that if both parties intended to enter into an agreement at the time they entered into negotiations for such an agreement then their intentions should be respected even though no actual exchange of money or goods occurred during those negotiations. Furthermore, he argued that since neither party had any knowledge of the applicable state law at the time they negotiated their agreement, enforcing its provisions would not serve any legitimate purpose but rather would only punish them for failing to comply with something they did not know existed in advance.