Since 1961, the Supreme Court has held that evidence illegally obtained by the state is not admissible in court under what is called the "exclusionary rule." The rule is designed to deter police misconduct and protect citizens from unwarranted searches and seizures. Beginning with the Rehnquist Court, however, the Court has been carving out exceptions. The most important and problematic of these exceptions is the "good faith" exception, which holds that evidence can't be thrown out if police officers obtained it in good faith, even if the search was illegal because of errors by other actors.
Today, the Court has found yet another exception. Police had conducted a search that was legal at the time under an 11th Circuit precedent, but was later ruled to be a violation of the Fourth Amendment. In an opinion by Justice Alito, the Court held that the evidence was still admissible under the good-faith exception -- the protections of the "exclusionary rule" cannot be applied retroactively. The problem, as Breyer's dissent notes, is that (in an increasing Roberts Court tradition) this leaves someone whose rights have been violated without a remedy. Ominously, this was a 7-2 decision, with neither of Obama's appointments joining Breyer and Ginsburg in dissent.
