Jury nullification
In the 1895 Supreme Court decision Sparf v. United States the Court ruled that judges did not have an obligation to tell jurors about jury nullification. The decision did not say judges could not tell jurors about nullification or that jurors did not have that power. They only said judges were not required to tell jurors. This led to the now common practice of judges in the U.S. penalizing defense attorneys who try in any way to present a nullification argument.