What is the test for appeal of Rule 52(b)?

"Rule 52(b) was intended to afford a means for the prompt redress of miscarriages of justice. By its terms, recourse may be had to the Rule only on appeal from a trial infected with error so 'plain' the trial judge and prosecutor were derelict in countenancing it, even absent the defendant's timely assistance in detecting it. The Rule thus reflects a careful balancing of our need to encourage all trial participants to seek a fair and accurate trial the first time around against our insistence that obvious injustice be promptly redressed."

United States v. Frady, 456 U.S. 152, 163 (1982).