In what circumstances will the US Supreme Court allow the federal government to commit crimes in order to secure the conviction of a private criminal?

In a government of laws, existence of the government will be imperilled if it fails to observe the law scrupulously. Our Government is the potent, the omnipresent teacher. For good or for ill, it teaches the whole people by its example. Crime is contagious. If the Government becomes a lawbreaker, it breeds contempt for law; it invites every man to become a law unto himself; it invites anarchy. To declare that in the administration of the criminal law the end justifies the means -to declare that the Government may commit crimes in order to secure the conviction of a private criminal -would bring terrible retribution. Against that pernicious doctrine this court should resolutely set its face.

Olmstead v. United States, 277 U.S. 438, 485 (1928) (Brandeis, J., dissenting).