Arizona v. Fulminante

Fulminante (defendant) confessed to murder to a fellow inmate who was a paid FBI informant, motivated by the informant's offer of protection from other inmates amid rumors he'd molested a young girl; he made a second, more detailed confession to the informant's wife shortly after, and the state supreme court found the first confession coerced but initially deemed its admission harmless error, then reversed course to hold harmless-error analysis inapplicable to coerced confessions at all.