A state enacts a law that retroactively criminalises an act that was legal when it was committed. This violates the rule of law principle known as

Nullum crimen sine lege means 'no crime without law,' which prohibits retroactive criminal legislation—punishing someone for an act that was not a crime when it was committed. Nemo judex in causa sua means no one should be a judge in their own case; audi alteram partem requires that both sides be heard; and res judicata refers to a matter already decided by a court.