In Harrington v. California, the U.S. Supreme Court ruled on a case involving the Sixth Amendment's right to confront witnesses. The defendant, Harrington, was convicted of first-degree murder based partly on testimonies from co-defendants who did not appear in court and thus could not be cross-examined by his attorney. On appeal, he argued that this violated his constitutional rights under the Confrontation Clause of the Sixth Amendment which guarantees defendants an opportunity to challenge their accusers in court. However, the Supreme Court upheld his conviction with a 5-3 decision stating that while it was error to admit these statements into evidence without giving him an opportunity for cross-examination; such error can be considered harmless if there is overwhelming independent evidence supporting guilt beyond reasonable doubt - as was deemed present in this case.

In the dissenting opinion for Harrington v. California, Justice William O. Douglas argued that the admission of a co-defendant's confession at trial violated the defendant's Sixth Amendment right to confront witnesses against him. He believed that even if there was overwhelming evidence of guilt, this did not justify ignoring constitutional rights and protections. In his view, it is impossible to measure how much weight a jury gives to an unconstitutional piece of evidence when determining guilt or innocence; therefore, any violation should result in automatic reversal regardless of other incriminating evidence presented at trial. Douglas feared that allowing such violations could lead down a slippery slope where more serious infringements on defendants' rights might be tolerated in future cases.