It's the well-known Donziger prosecution, which involved a lawyer being held in contempt. Justice Kavanaugh joined the dissent, which explains:
When Mr. Donziger failed to comply fully with the court’s orders, it held him in criminal contempt and referred the matter to the U. S. Attorney’s Office for prosecution. See 38 F. 4th 290, 295 (CA2 2022). After some deliberation, however, the U.S. Attorney “‘respectfully declined’” to take up the case. Ibid. (alteration omitted).
Apparently displeased with this decision, the district court responded by setting up and staffing its own prosecutor’s office. Ibid. In the bench trial that followed, that office secured a conviction and the court sentenced Mr. Donziger to six months in prison. Ibid. Throughout these proceedings and on appeal, Mr. Donziger objected. He argued that the district court had no lawful authority to override the Executive Branch’s nonprosecution decision and that our Constitution’s separation of powers exists in no small measure to keep courts from becoming partisans in the cases before them. Despite his arguments, the Second Circuit affirmed Mr. Donziger’s conviction. Id., at 306. Judge Menashi dissented. Id., at 306–315.
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However much the district court may have thought Mr. Donziger warranted
punishment, the prosecution in this case broke a basic constitutional
promise essential to our liberty. In this country, judges have no more
power to initiate a prosecution of those who come before them than
prosecutors have to sit in judgment of those they charge. In the name
of the “United States,” two different groups of prosecutors have asked
us to turn a blind eye to this promise. Respectfully, I would not. With
this Court’s failure to intervene today, I can only hope that future
courts weighing whether to appoint their own prosecutors will consider
carefully Judge Menashi’s dissenting opinion in this case, the
continuing vitality of Young, and the limits of its reasoning. Our Constitution does not tolerate what happened here.