The U.S. Supreme Court case Terry Williams v. John Taylor, Warden in 1999 revolved around the issue of whether a defendant's constitutional rights were violated due to ineffective assistance of counsel during sentencing proceedings. Terry Williams was convicted for murder and sentenced to death in Virginia state court. His lawyers failed to present significant mitigating evidence about his background during the penalty phase, including severe physical and sexual abuse he suffered as a child which could have influenced his behavior leading up to the crime committed. Williams appealed on grounds that this failure constituted ineffective assistance of counsel under Strickland v Washington (1984), violating his Sixth Amendment right to fair representation by competent legal counsel. The Supreme Court agreed with him, ruling that even though there was overwhelming evidence supporting guilt, failing to introduce such crucial mitigating information at sentencing amounted to constitutionally deficient performance by defense attorneys. This decision underscored the importance of effective legal representation not just during trial but also at sentencing hearings where life or death decisions are made.

In the dissenting opinion for Terry Williams v. John Taylor, Warden, Justice Scalia disagreed with the majority's decision to grant habeas relief based on ineffective assistance of counsel during sentencing. He argued that there was no reasonable probability that a different outcome would have occurred if additional mitigating evidence had been presented at trial. The justice believed that the brutal nature of Williams' crimes outweighed any potential sympathy jurors might feel due to his troubled upbringing and mental health issues. Furthermore, he contended that it was not clear whether presenting such information would have helped or hurt Williams' case since it could also provide further insight into his violent tendencies. Therefore, in Scalia’s view, even though defense counsel may not have pursued all possible avenues for mitigation evidence during sentencing phase preparation and presentation, this did not necessarily constitute deficient performance warranting reversal under Strickland v Washington standards.