Coan v. Flagg 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 habeas corpus to a prisoner who was being held in a federal prison. The case arose when a prisoner, William Coan, was held in a federal prison in New York. Coan sought a writ of habeas corpus from the New York state court, claiming that he was being held in violation of his constitutional rights. The federal government argued that the state court did not have the authority to issue the writ, as the federal government had exclusive jurisdiction over federal prisons. The Supreme Court ultimately held that the state court did not have the authority to issue the writ of habeas corpus. The Court reasoned that the federal government had exclusive jurisdiction over federal prisons, and that the state court did not have the authority to interfere with the federal government's authority. The Court also noted that the writ of habeas corpus was a remedy that was available only to those who were held in state custody, and not to those held in federal custody. As such, the Court held that the state court did not have the authority to issue the writ of habeas corpus to Coan.

Justice Field delivered the dissenting opinion in Coan v. Flagg, a case concerning whether or not an individual could be held liable for damages caused by another person's negligence. Justice Field argued that the majority had erred in their decision to hold the defendant responsible for damages caused by another party’s negligence. He reasoned that since there was no contract between them and they were not acting as agents of each other, it would be unjust to impose liability on one party when neither had any control over the actions of the other. Furthermore, he noted that if such a rule were adopted then individuals who have nothing to do with an accident would still be held liable simply because they happened to own property near where it occurred. In conclusion, Justice Field believed that holding someone accountable for damage done without their knowledge or consent was contrary both to justice and common sense and should therefore not be allowed under law.