Williams was convicted in California of multiple counts of robbery and kidnap for robbery, and sentenced under the amended version of Penal Code section 654, which required the court to impose the sentence for a count carrying a higher sentence when multiple counts were based upon the same act. Under the version of section 654 in effect at the time Williams committed the offenses, the trial court had the discretion to impose either sentence for two counts based on the same act. Reversing the district court’s denial of a habeas petition, the appellate court held that the state court’s application of an amended version of section 654, eliminating judicial discretion to impose a lower sentence afforded by the version in place at the time of the petitioner’s offense, violated the Ex Post Facto Clause because it disadvantaged the petitioner by lengthening the time he would spend in prison. The court held that this error required reversal without inquiring into its harmfulness because ex post facto sentencing violations are not “trial-type” errors subject to harmless error analysis under Brecht v. Abrahamson (1993) 507 U.S. 619.