In the case of Eddie S. Ylst, Warden v. Owen Duane Nunnemaker (1990), the U.S. Supreme Court addressed how federal courts should handle state court decisions when considering habeas corpus petitions from prisoners who claim their constitutional rights were violated during state proceedings. The petitioner, Nunnemaker, had been convicted of murder in California and his conviction was upheld by a state appeals court despite his claims that he received ineffective assistance from his trial counsel due to a conflict of interest. He then filed for federal habeas relief on the same grounds but was denied because it appeared he had not exhausted all available remedies at the state level as required by law before seeking federal intervention. The Supreme Court held that when there is ambiguity about whether a prisoner's constitutional claim has been adjudicated on its merits or dismissed based on procedural default in lower courts, federal judges must presume that it was decided on procedural grounds unless there is clear evidence to suggest otherwise - known as "Ylst presumption". This ruling clarified an important aspect of how federal courts review cases involving potential violations of defendants' constitutional rights during criminal trials conducted under state laws.

In the case of Eddie S. Ylst, Warden v. Owen Duane Nunnemaker, Justice Marshall dissented from the majority opinion. He argued that federal courts should not be required to guess what a state court might have decided on an issue if it had been presented with one in order to determine whether or not a procedural default occurred at the state level. Instead, he believed that federal courts should only consider actual decisions made by state courts and base their judgments on those facts alone. In his view, this approach would respect both states' rights and defendants' rights while also promoting judicial efficiency by avoiding unnecessary speculation about hypothetical scenarios.