In Alvarado v. United States (1989), the Supreme Court ruled on whether a defendant's Sixth Amendment right to counsel was violated when his attorney failed to object to an erroneous jury instruction regarding the burden of proof for insanity defense. The case involved Francisco Alvarado, who had been convicted of bank robbery and murder in federal court. His lawyer did not challenge a flawed jury instruction that stated it was up to the defendant, rather than the prosecution, to prove he was insane at the time of his crimes - contrary to established law which places this burden on prosecutors. The Supreme Court held that even though there had been error in instructing jurors about how they should consider evidence related with insanity defense, such mistake didn't necessarily mean ineffective assistance from counsel or violation of Sixth Amendment rights. The justices reasoned that strategic decisions made by attorneys during trial are generally immune from hindsight scrutiny unless no competent counsel would have taken such action.

In the dissenting opinion for Alvarado v. United States, Justice Brennan argued that the majority's decision to uphold Alvarado's conviction was a departure from established precedent regarding the admissibility of evidence obtained through unlawful searches and seizures. He contended that by allowing illegally seized evidence to be used in court because it would have been inevitably discovered, the Court undermined Fourth Amendment protections against unreasonable searches and seizures. Furthermore, he criticized the majority’s application of this “inevitable discovery” exception as speculative and subjective since there was no clear indication that law enforcement would have found this evidence without violating Alvarado’s rights. In his view, such an approach could potentially encourage police misconduct by providing them with a loophole to bypass constitutional safeguards if they can later argue that any unlawfully obtained evidence would have eventually been discovered legally.