The English Exchequer Rule, which was adopted by American courts, required presumption of prejudice and virtually automatic reversals of convictions upon showing of error. This created backlogs and delays and thus was replaced by the 1919 harmless error statute which stated that if an error did not affect the defendant's substantial rights, it could be adjudged harmless. The Supreme Court interpreted this statute to include most errors not violating explicit constitutional rights. Moreover, the defendant has the burden of proving that the error affected his or her substantial rights, resulting in prejudice. The objective of rules that regulate prosecutoral and judicial conduct at trial is to shield the jury from matters upon which no evidence can be offered and which are likely to influence the verdict. For example, it is an error for the prosecutor to overtly appeal to jurors' passions or prejudices or to comment on the accused's silence. Likewise, the trial judge must not display hostility toward a defense counsel. Prosecutorial errors often occur because the prosecutor is trying unconditionally to secure a conviction in an adversarial system. However, courts disapprove of such conduct but uphold convictions or apply the harmless error standard and instruct the jury to disregard the prosecutor's conduct. Consequently, courts do not induce prosecutors to comply with trial rules governing their behavior and encourage systematic erosion of justice in the form of a high incidence of nontrivial errors. The automatic reversal standard would halt this trend and promote more rational verdicts. The paper includes 141 footnotes.