In Perez v. California, the U.S Supreme Court ruled in 1968 that a defendant's right to due process was violated when he was not allowed to cross-examine a key prosecution witness about his pending felony charges. The case involved Manuel Perez, who had been convicted of murder based on testimony from an accomplice named Alcala. During the trial, Perez's lawyer tried to question Alcala about whether he expected leniency on his own pending charges in exchange for testifying against Perez but was prevented by the judge from doing so. In its decision, the Supreme Court held that this restriction denied Perez his constitutional right under the Sixth Amendment to confront witnesses against him and ordered a new trial.

In the dissenting opinion for Perez v. California, Justice Douglas argued that the defendant's constitutional rights were violated due to an improper jury instruction regarding reasonable doubt. He believed that the trial court erred in instructing jurors they could find guilt based on a preponderance of evidence rather than beyond a reasonable doubt - which is constitutionally required in criminal cases. This, he contended, lowered the prosecution’s burden of proof and potentially influenced the jury's decision-making process against Perez. Furthermore, he emphasized that this case was not about whether there was sufficient evidence to convict but rather if proper legal standards were applied during deliberation by jurors. Thus, while acknowledging Perez may have been guilty as charged under correct instructions; he insisted it was crucial for courts to uphold defendants' constitutional protections regardless of their perceived guilt or innocence.