The United States v. Mechanik et al., 1985, is a Supreme Court case that revolved around the issue of whether an error in grand jury proceedings should lead to dismissal of an indictment if the defendants were subsequently convicted by a petit jury. The respondents, including Dr. Michael J. Mechanik, were indicted for conspiracy to distribute and possess controlled substances based on evidence presented before a grand jury where two witnesses testified simultaneously - which was against Federal Rule of Criminal Procedure 6(d). They did not raise this concern until after they had been convicted at trial but appealed their convictions arguing that the simultaneous testimony invalidated their indictments. The Supreme Court held in favor of the government with Justice O'Connor delivering the opinion stating that any error committed during grand jury proceedings was rendered harmless beyond reasonable doubt by subsequent guilty verdicts returned by separate and independent juries (petit juries). Therefore, such errors do not automatically require reversal or dismissal post-conviction unless they influenced the final outcome.

In the dissenting opinion for United States v. Mechanik, Justice Stevens argued that the majority's decision to uphold a conviction despite procedural errors in grand jury proceedings undermined the integrity of those proceedings and violated defendants' rights. He contended that such errors should not be dismissed as harmless simply because they did not affect the petit jury’s verdict. Instead, he believed any error affecting grand jury deliberations was significant enough to warrant reversal of a conviction, regardless of its impact on trial outcome. This view is grounded in his belief that every defendant has an absolute right to have charges reviewed by a properly constituted grand jury before facing trial or punishment; this right cannot be compromised even if it does not ultimately influence their guilt or innocence determination at trial.