In Thompson v. Missouri, the U.S. Supreme Court ruled on whether a state could enact laws that would allow previously inadmissible evidence to be used in criminal trials and apply them retroactively. The case arose when Mr. Thompson was convicted of murder based on new legislation allowing documents into evidence that were not admissible at the time of his crime or initial trial. On appeal, he argued this violated the ex post facto clause of the Constitution which prohibits states from passing laws that change punishment for a crime after it has been committed. The court held 6-2 that such laws did not violate this constitutional provision as they neither made an action illegal which was legal when done nor aggravated a crime already committed; rather, they simply altered rules of evidence to admit facts relevant to guilt or innocence more effectively than before their enactment.

In the dissenting opinion for Thompson v. Missouri, Justice John Marshall Harlan argued that the admission of evidence obtained after a crime has been committed and after an indictment has been issued should not be permitted if such evidence was not admissible at the time of trial under existing laws. He contended that this violated ex post facto laws which prohibit retroactive application of criminal law to disadvantage a defendant. Harlan believed it was unfair to allow changes in evidentiary rules to apply retrospectively as it could potentially subject individuals to penalties they would have avoided under previous regulations. Thus, he disagreed with the majority's view that amendments allowing new types of evidence did not fall within ex post facto prohibitions because they didn't alter "substantial personal rights" but only affected court procedures.