In the case of Saul Ornelas and Ismael Ornelas-Ledesma v. United States, 1995, the U.S Supreme Court had to determine whether a police officer's decision to conduct a search without warrant was reasonable under Fourth Amendment standards. The defendants were stopped by Milwaukee police officers for a traffic violation. After observing their nervous behavior and other suspicious circumstances, including an alleged false statement about their travel plans, the officers conducted a search of their car where they found cocaine. The defendants argued that this evidence should be suppressed because it was obtained through an unreasonable search in violation of the Fourth Amendment. The Supreme Court held that appellate courts must use de novo review when determining if an officer’s suspicion justifies a warrantless search rather than using clear error or substantial evidence standard reviews which are more deferential to trial court findings. This means that higher courts can fully reconsider lower court decisions on these matters instead of only being able to overturn them due to obvious errors or lack of supporting evidence.

In the dissenting opinion for Saul Ornelas and Ismael Ornelas-Ledesma v. United States, Justice Scalia disagreed with the majority's decision to apply a de novo standard of review to determinations of reasonable suspicion and probable cause. He argued that this approach would undermine the authority of trial courts and lead to inconsistent rulings across different jurisdictions. Instead, he advocated for an abuse-of-discretion standard which gives more deference to lower court decisions unless they are clearly erroneous or unreasonable. According to Scalia, such a standard respects the expertise and firsthand perspective of trial judges who are in a better position than appellate judges to evaluate evidence and make factual findings about police conduct during searches and seizures.