The great majority of trial errors are subject to harmless error review. See Fed.R.Crimp.P. 52(a). Some errors, known as structural errors, are not. We are reminded of this with the Supreme Court's per curiam decision this week in Glebe v. Frost. In that case, the Ninth Circuit below held that a state trial court's restriction of defense counsel's closing argument was structural error that required a new trial. Ninth Circuit law is clear on this. The problem for the defendant is that Supreme Court precedent is not. In other words, the Supreme Court has never held that restricting defense counsel's closing argument amounts to structural error. Because this was a habeas case, and because the asserted right was not clearly established by decisions from the Supreme Court, the defendant ultimately lost.
This was not the greatest case in terms of establishing the impropriety of harmless error review. The defendant thrice confessed prior to trial, then took the stand at trial and confessed a fourth time. Defense counsel, in closing, sought to challenge the sufficiency of the evidence, as well as assert a duress defense. The latter argument might make sense; the former strains credulity. In any event, the state trial court told defense counsel to pick one defense and prohibited any alternative arguments. The attorney did the obvious and argued duress. On appeal, the Washington State Supreme Court held that the restriction on closing argument was error, but that the error was harmless.
So, as it stands now, the Washington Supreme Court says such restrictions on closing argument are harmless error (according to the State's petition, most other courts do as well); the Ninth Circuit says that such restrictions are structural errors not subject to harmless error analysis. The issue is an open one in the Supreme Court. As far as we can tell, it is an open question in the Tenth Circuit as well.
Moving forward, remember that structural errors are those that infect the entire trial process and render it fundamentally unfair (such as the denial of counsel, a biased judge, and other reasons mentioned here). And you want errors to be structural. Otherwise, the error could be harmless. Some errors obviously are not structural; if it is not obvious, make the argument.