In the case of Williams v. Johnson, 2013, Marvin Pete Walker Jr., a California death row inmate who had been convicted for murder and robbery in 1980, challenged his conviction on grounds that he received ineffective assistance from his counsel during trial. He claimed that his attorney failed to investigate and present mitigating evidence about his background during the penalty phase of trial which could have potentially spared him from a death sentence. The Ninth Circuit Court initially granted relief but was reversed by the Supreme Court because it did not defer to reasonable state court adjudications on federal law issues as required under federal law (28 U.S.C §2254(d)). The Supreme Court held that there was no clear proof that state court's decision was contrary to or involved an unreasonable application of clearly established Federal Law.

In the dissenting opinion for Williams v. Johnson, it was argued that the majority's decision to uphold a lower court ruling denying habeas corpus relief to a convicted murderer failed to properly apply federal law governing such appeals. The dissent contended that the state court had unreasonably applied clearly established Federal law in rejecting petitioner’s claim of ineffective assistance of counsel during his trial and sentencing phase. They believed there were significant errors made by defense counsel which could have influenced the outcome of both guilt and penalty phases, including failing to investigate or present substantial mitigating evidence at sentencing about defendant's background and mental health issues. This failure, they argued, amounted to deficient performance under Strickland v. Washington standard for evaluating claims of ineffective assistance of counsel - an unreasonable application of Supreme Court precedent by California courts.