In Dixon et al. v. United States, the Supreme Court examined whether a federal court in Texas had jurisdiction over a case involving an alleged violation of the Mann Act, which prohibits transporting individuals across state lines for immoral purposes. The defendants were accused of taking two women from Florida to Texas with intent to engage in prostitution and argued that since they did not cross state lines while within Texas, the federal court lacked jurisdiction. However, the Supreme Court disagreed and upheld their convictions by ruling that crossing any state line at any point during travel is sufficient for establishing federal jurisdiction under the Mann Act.

In the dissenting opinion for Dixon et al. v. United States, Justice Goldberg argued that the majority's interpretation of Rule 30 was too narrow and failed to consider its broader purpose: ensuring a fair trial by providing clear instructions to juries about how they should apply the law in their deliberations. He contended that this rule is not merely procedural but also substantive because it directly affects defendants' rights and potential outcomes of trials. In his view, when a judge refuses or fails to provide requested instructions on key legal principles relevant to a case—such as self-defense in this instance—it can confuse jurors and lead them astray from proper considerations under law, thereby undermining justice. Therefore, he believed such errors should be considered harmful unless proven harmless beyond reasonable doubt—a standard which wasn't met here according to him—and thus warrant retrials rather than being dismissed as trivial or inconsequential.