Lammon & Others v. Feusier & Others 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 mandamus to a federal court. The case involved a dispute between two parties over a contract for the sale of land. The state court had issued a writ of mandamus to the federal court, ordering it to issue a judgment in favor of the plaintiff. The Supreme Court held that the state court did not have the authority to issue a writ of mandamus to a federal court. The Court reasoned that the state court lacked the power to issue a writ of mandamus to a federal court because the federal court was a separate and independent court. The Court also noted that the state court had no authority to interfere with the proceedings of the federal court. The Court concluded that the state court had exceeded its authority in issuing the writ of mandamus and that the writ was therefore invalid.

In the case of Lammon & Others v. Feusier & Others, the Supreme Court was asked to decide whether a contract between two parties should be enforced despite one party's failure to comply with certain conditions that were part of the agreement. The majority opinion held that since there had been substantial performance on both sides, and no fraud or mistake in entering into the contract, it should be enforced as written. Justice Field dissented from this decision and argued that while contracts are generally binding when they have been entered into without fraud or mistake, exceptions can still exist if enforcing them would lead to an unjust result. In this particular case he felt that allowing enforcement would cause an injustice because one party had failed to fulfill their obligations under the agreement and thus could not reasonably expect its benefits. He concluded by stating his belief that such cases must be decided based on principles of equity rather than strict adherence to legal rules alone.