In the 2001 case of Walter Mickens, Jr. v. John Taylor, Warden, the U.S Supreme Court ruled that a defendant must demonstrate that a conflict of interest affected his counsel's performance to establish a violation of his Sixth Amendment right to effective assistance of counsel. The case involved Walter Mickens who was convicted for murder and sentenced to death in Virginia state court. His attorney had previously represented the victim in an unrelated assault and battery charge but failed to disclose this fact or seek Mickens' consent before representing him at trial. Upon discovering this information post-conviction, Mickens appealed on grounds that he received ineffective assistance due to conflict of interest by his lawyer which violated his Sixth Amendment rights. However, the Supreme Court held that mere possibility of conflict is not sufficient; rather it needs proof showing adverse effect on representation.

In the dissenting opinion for Walter Mickens, Jr. v. John Taylor, Warden (2001), Justice Stevens argued that a defendant's right to counsel is so fundamental to ensuring fair trials that any breach of this constitutional guarantee should automatically result in reversal of conviction without requiring proof of prejudice. He disagreed with the majority's application of harmless-error analysis and insisted on strict adherence to Sullivan v. Louisiana which held that certain errors are structural and thus not subject to such analysis. In his view, when an attorney has a conflict as severe as representing both victim and accused in related cases, it undermines the integrity of criminal proceedings enough to be considered structural error warranting automatic reversal.