McKnight v. United States was a United States Supreme Court case that dealt with the issue of whether a defendant could be convicted of a crime if the evidence presented was obtained through an illegal search and seizure. The case involved a man named McKnight who was charged with receiving stolen goods. The evidence used to convict him was obtained through a search of his home without a warrant. The Supreme Court ruled in favor of McKnight, holding that the evidence obtained through the illegal search and seizure was inadmissible in court. The Court reasoned that the Fourth Amendment of the United States Constitution protects citizens from unreasonable searches and seizures, and that the evidence obtained through the illegal search and seizure was in violation of this amendment. The Court also held that the exclusionary rule applied in this case, meaning that any evidence obtained through an illegal search and seizure was inadmissible in court. The decision in McKnight v. United States established the precedent that evidence obtained through an illegal search and seizure is inadmissible in court, and that the exclusionary rule applies in such cases. This decision has been cited in numerous cases since then, and has been an important part of Fourth Amendment jurisprudence.

Justice Field delivered the dissenting opinion in McKnight v. United States, arguing that the majority's decision was contrary to established precedent and would lead to an unjust result. He argued that if a person is charged with a crime, they should be able to present evidence of their innocence at trial even if it contradicts prior statements made by them or others during plea negotiations. In this case, he noted that there had been no formal agreement between the parties and thus any admissions made by defendant were not binding on him as part of such an agreement. Furthermore, Justice Field argued that allowing prosecutors to use these admissions against defendants would have a chilling effect on plea bargaining since defendants could never be sure what might later be used against them in court proceedings. As such, he concluded that it was wrong for the Court to deny defendant his right to present evidence of his innocence at trial simply because he had previously admitted guilt during informal discussions with prosecutors outside of court proceedings.