In the case of Sigler, Warden v. Parker in 1969, the U.S Supreme Court ruled on a habeas corpus petition from Robert Glen Parker who was convicted for murder and sentenced to death by an Nebraska court. The main issue revolved around whether or not Parker's constitutional rights were violated when he wasn't allowed to cross-examine a co-defendant who had given a confession implicating him in the crime. The Supreme Court held that there was no violation of his Sixth Amendment right to confront witnesses against him because his co-defendant’s confession corroborated with other evidence presented at trial independently of it. Therefore, even though he could not cross-examine this witness (the co-defendant), it did not undermine confidence in his conviction since there was substantial independent evidence supporting it.

In the dissenting opinion for Sigler, Warden v. Parker, Justice Harlan disagreed with the majority's decision to grant habeas corpus relief to a state prisoner who had been denied due process because of an allegedly prejudiced jury. He argued that there was no clear evidence showing that the juror in question was actually biased against the defendant or that she influenced other jurors' decisions. Furthermore, he contended that it is not enough for a defendant to simply allege bias; they must provide substantial proof of actual prejudice which affects their right to a fair trial. Additionally, Justice Harlan expressed concern over federal courts interfering too much in state criminal proceedings and undermining states' rights and responsibilities in administering justice within their jurisdictions.