In interview with Adam Liptak:

The court’s “second-biggest error,” Justice Stevens said, was the Citizens United campaign finance decision. “Money in politics — it’s hard to believe the extent of it,” he said.

The third-biggest mistake, he said, was Bush v. Gore, the 2000 decision that handed the presidency to George W. Bush. “It was really a disgrace,” he said.

(The other case Justice Stevens flagged was the Heller gun case).