In Tennessee, a confessed killer is free again after the state Supremes voided his conviction. Why? Because police questioned the guy without reading him the Miranda rights first. Once he confessed, THEN they read him his rights and, using his previous statements, got him to confess again. It's a practice called "question first," and it's unconstitutional.
("Question first" was actually part of a Supreme Court case that ties back to a Missouri arson.)
He was behind bars four years for strangling a woman. Theoretically, police could try to convict him again. But it sounds like the odds are rather slim.