In the case of Sumner, Warden v. Mata (1981), the U.S. Supreme Court ruled that federal courts must give deference to state court decisions on issues of fact but not on matters of law or mixed questions of law and fact. The petitioner, a prison warden, appealed a decision by the Ninth Circuit Court which granted habeas corpus relief to respondent Mata based on his claim that he had been denied effective assistance of counsel during his trial in state court for murder charges. The Supreme Court held that while the Federal Habeas Corpus Act requires federal courts to respect findings made by state courts regarding factual disputes unless they are clearly erroneous, it does not require such deference when it comes to legal conclusions or determinations involving both facts and laws. Therefore, although there was sufficient evidence supporting the finding that Mata's attorney did not adequately investigate potential defenses before advising him to plead guilty - this alone did not establish ineffective assistance under prevailing constitutional standards.

In the dissenting opinion for Sumner v. Mata, Justice William Rehnquist argued that the majority's decision undermined the principles of federalism and comity by allowing federal courts to reevaluate state court findings without sufficient cause. He contended that under 28 U.S.C §2254(d), a presumption of correctness should be applied to factual determinations made by state courts in habeas corpus proceedings unless one of seven specific conditions is met. In this case, he believed none were applicable; hence, there was no reason to question California Supreme Court’s conclusion on Mata's competency at trial or his attorney’s effectiveness. Furthermore, he criticized the majority for applying an overly broad interpretation of "unreasonable determination" clause which could lead to unnecessary interference with state court decisions in future cases.