In the case of Shinn v. Kayer, 2020, the U.S Supreme Court reversed a Ninth Circuit decision that had granted habeas relief to an Arizona death-row inmate named James Erin McKinney. The central issue was whether McKinney's post-conviction counsel provided ineffective assistance by failing to investigate and present substantial mitigating evidence during his penalty phase re-sentencing proceedings in state court. The Ninth Circuit held that he did, but the Supreme Court disagreed with this conclusion. The majority opinion stated that under federal law governing review of state convictions (28 U.S.C §2254(d)), a federal court can only grant habeas relief if the state-court decision was contrary to or involved an unreasonable application of clearly established Federal law as determined by the Supreme Court itself. In this case, it found no such violation because there wasn't any clear precedent from its prior decisions which required defense counsel in capital cases to investigate all potentially mitigating evidence regardless of their strategic considerations. Therefore, according to SCOTUS' ruling, even though McKinney’s lawyer didn’t fully explore potential mitigation arguments before deciding not to use them at sentencing - which might have been considered deficient performance under normal circumstances - it couldn’t be said they were objectively unreasonable given existing legal standards.

In the dissenting opinion for SHINN v. KAYER, Justice Alito, joined by Justices Thomas and Gorsuch, argued that the Ninth Circuit Court of Appeals overstepped its authority in granting habeas relief to Kayer. They contended that the lower court's decision was based on a de novo review instead of adhering to the Antiterrorism and Effective Death Penalty Act (AEDPA) standard which requires deference to state-court decisions unless they are contrary or involve an unreasonable application of clearly established federal law. The dissenters believed that there was no clear evidence showing that Kayer’s counsel performed deficiently during his trial or sentencing phase as required under Strickland v. Washington precedent for ineffective assistance claims. Furthermore, they asserted it wasn't proven beyond reasonable doubt whether competent representation would have led to a different outcome at sentencing given substantial aggravating factors against Kayer.