In Tanner et al. v. United States (1986), the U.S Supreme Court ruled that a jury's verdict cannot be impeached by post-trial testimony about juror misconduct during deliberations, even if it involves substance abuse. The case involved defendants who were convicted of mail fraud and racketeering charges related to an alleged scheme to defraud their employer and its insurance carrier by staging fake accidents at work sites. After the trial, two jurors came forward with allegations that several members of the jury had consumed alcohol, smoked marijuana, used cocaine and slept through parts of the trial proceedings. The defense sought a new trial on these grounds but was denied by both district court and appellate court rulings which held that Federal Rule of Evidence 606(b) prohibits inquiry into internal influences affecting jurors' decision-making process unless extraneous prejudicial information was improperly brought to bear upon any juror or there was outside influence improperly brought upon any juror.

In the dissenting opinion for Tanner et al. v. United States, Justice O'Connor argued that the majority's decision to uphold a strict interpretation of Federal Rule of Evidence 606(b) was incorrect and overly restrictive. This rule prohibits jurors from testifying about their deliberations during an inquiry into the validity of a verdict or indictment unless it involves extraneous prejudicial information or outside influence improperly brought to bear on any juror. The dissent believed this case presented exceptional circumstances where evidence regarding alcohol and drug use by jurors during trial should have been admissible in order to ensure fairness and integrity in judicial proceedings. They contended that such behavior constituted "outside influence," as it introduced factors not part of the evidence or law into deliberations, thus potentially compromising impartiality and accuracy in reaching a verdict.