In the case of Weaver v. Graham, Governor of Florida in 1980, the U.S. Supreme Court ruled that a Florida statute reducing "gain time" for good behavior in prison violated the Ex Post Facto Clause of the Constitution. The clause prohibits laws from being enacted that increase punishment for a crime after it has been committed. In this case, Thomas Earl Weaver was sentenced to prison before the new law took effect and argued that his sentence had effectively been increased by its application to him because he would serve more time than under previous rules governing gain time credits. The court agreed with Weaver's argument and held that even though no one is constitutionally entitled to early release through gain-time credits, once such benefits are established as part of an inmate’s sentence they cannot be reduced retroactively without violating ex post facto prohibitions.

In the dissenting opinion for Weaver v. Graham, Justice Rehnquist argued that the majority's decision was based on a misunderstanding of ex post facto laws. He contended that changes in parole guidelines did not constitute an increase in punishment but were merely adjustments to the method of determining early release eligibility. The new law did not guarantee more severe punishment; it only altered how and when such decisions would be made, which is within the state's rights. Furthermore, he pointed out that prisoners could still earn gain time under other provisions even if they lost some opportunities due to this change. Therefore, he believed there was no violation of constitutional principles as claimed by the majority.