In Greer, Warden v. Miller (1986), the U.S Supreme Court ruled in favor of the petitioner, Greer, who was a prison warden. The respondent, Miller had been convicted for murder and sentenced to death by an Illinois court but argued that his constitutional rights were violated during sentencing because he was not allowed to present mitigating evidence about his character and background. However, the Supreme Court held that there was no violation as Miller's counsel did have ample opportunity to present such evidence at trial but chose not to do so. Furthermore, it found that even if this right had been denied at sentencing stage it would be harmless beyond reasonable doubt since all relevant mitigating factors were presented before jury during guilt phase of trial.

In the dissenting opinion for Greer, Warden v. Miller, Justice Brennan disagreed with the majority's view that a harmless error analysis could be applied to this case. He argued that such an analysis was inappropriate because it allowed the court to speculate about what might have happened if the jury had been properly instructed on how to consider evidence of prior convictions in their deliberations. In his view, there was no way to know whether or not these instructions would have changed the outcome of Miller’s trial and therefore it was impossible to say that their absence constituted a harmless error. Furthermore, he believed that applying this type of analysis undermined defendants' rights by allowing courts too much discretion in determining which errors were harmful enough to warrant reversal and which were not.