The US Foreign Intelligence Surveillance Court (FISC) is the secret authority that judges on whether or not surveillance actions conducted under programs like NSA's PRISM are constitutional. While the FISC has been accused of being a rubber stamp for the government, approving nearly all of its surveillance requests, it hasn't always been that way.
A secret court opinion from October 2011 that ruled the NSA's surveillance activities unconstitutional has finally been unveiled, thanks to a successful challenge by the Electronic Frontier Foundation. (Big internet high five to the EFF by the way.) The document is heavily redacted, but as it hinges on the NSA's data collection methods, it offers interesting insight.
For the rest of the story: http://motherboard.vice.com/blog/the-fisa-court-knew-the-nsa-lied-repeatedly-about-its-spying-approved-its-searches-anyway