In the 1933 case Funk v. United States, the Supreme Court ruled on a matter concerning jury impartiality and conflict of interest. The defendant, Mr. Funk was convicted for conspiracy to violate Prohibition laws by a jury that included two brothers-in-law of an Assistant District Attorney who had participated in preparing and trying the case against him. The defense argued that this constituted juror bias due to familial relationship with a prosecuting attorney involved in the trial preparation but not present during actual court proceedings. The Supreme Court overturned Mr. Funk's conviction, ruling it as prejudicial error because these jurors were related within third degree (brother-in-law) to government counsel assisting in prosecution of accused which could potentially influence their decision making process or perception about guilt or innocence regardless if they are directly participating at trial stage or not. This landmark decision emphasized importance of maintaining absolute impartiality among jurors for fair administration of justice; any potential source causing reasonable doubt regarding this should be eliminated from outset ensuring every individual’s right to unbiased judgment under law is protected.

In the dissenting opinion for Funk v. United States, Justice McReynolds disagreed with the majority's decision to overturn a long-standing rule that spouses were incompetent as witnesses against each other in criminal cases. He argued that this was not an issue of constitutionality but rather one of policy best left to legislative bodies. He further contended that such a significant change should be based on compelling reasons and evidence, which he believed were lacking in this case. Moreover, he expressed concern about potential harm resulting from allowing spouses to testify against each other due to marital tensions or estrangements potentially leading to false testimonies. In conclusion, Justice McReynolds maintained his belief in preserving traditional legal principles unless there are strong justifications for changing them.