In McCarthy v. United States, the Supreme Court ruled that a defendant must be fully aware of the consequences before pleading guilty to a crime. The case involved an individual who pleaded guilty to tax evasion but later argued he was not adequately informed about potential penalties and his right to trial by jury. The court held that Rule 11 of the Federal Rules of Criminal Procedure requires courts to ensure defendants understand these implications prior to accepting their plea. If this process is not followed, any resulting conviction or sentence can be set aside on appeal. This decision emphasized the importance of protecting defendants' constitutional rights during legal proceedings and ensuring they are making informed decisions about their pleas.

In the dissenting opinion for McCarthy v. United States, Justice Harlan argued that the majority's decision to automatically reverse convictions without any showing of prejudice against defendants was an overextension of Rule 11. He believed this ruling would unnecessarily burden the judicial system with retrials and appeals based on technicalities rather than actual harm suffered by defendants. Instead, he proposed a more balanced approach where courts could examine each case individually to determine if a defendant’s rights were truly violated during plea proceedings. This way, only those who experienced substantial injustice would be granted new trials or pleas under Rule 11 violations.