In a recent opinion, Williams v. Pennsylvania, --- S.Ct. ----,  2016 WL 3189529 (June 9, 2016), a divided United States Supreme Court held that judges must recuse themselves in cases in which they previously played a significant role in prosecuting the person appearing before them. 

Petitioner Williams was convicted of the 1984 murder of Amos Norwood and sentenced to death. During the trial, the then-District Attorney of Philadelphia, Ronald Castille, approved the trial prosecutor's request to seek the death penalty against Williams. Over the next 26 years, Williams's conviction and sentence were upheld on direct appeal, state post-conviction review, and federal habeas review.

In 2012, Williams sought post-conviction relief arguing that the prosecutor had obtained false testimony from his codefendant and suppressed material, exculpatory evidence in violation of Brady v. Maryland, 373 U.S. 83 (1963). Finding that the trial prosecutor had committed Brady violations, the trial court stayed Williams's execution and ordered a new sentencing hearing. The Commonwealth asked the Pennsylvania Supreme Court, whose Chief Justice was former District Attorney Ronald Castille, to vacate the stay. Williams filed a response, along with a motion asking Justice Castille to recuse himself or, if he declined to do so, to refer the motion to the full court for decision. Without explanation, Justice Castille denied Williams's motion for recusal and the request for its referral. Justice Castille then joined the State Supreme Court opinion vacating the trial court's grant of penalty-phase relief and reinstating Williams's death sentence. Two weeks later, Justice Castille retired from the bench.

In a 5-3 ruling, the Supreme Court held that former Pennsylvania Supreme Court Justice Castille should not have considered the appeal by Williams because as a District Attorney years earlier Castille had made the decision to seek the death penalty against Williams.  According to Justice Kennedy, who wrote the opinion for the Supreme Court, "where a judge has had an earlier significant, personal involvement as a prosecutor in a critical decision in the defendant's case, the risk of actual bias in the judicial proceeding rises to an unconstitutional level."  Because Justice Castille's authorization to seek the death penalty against Williams amounts to significant, personal involvement in a critical trial decision, his failure to recuse from Williams's case presented an unconstitutional risk of bias and constituted "structural error even if the judge in question did not cast the deciding vote."

Cornell has posted a link to the opinion here.