Recently unsealed documents show that Microsoft was able to beat back a National Security Letter (NSL) from the FBI.
NSL are like subpoenas but go through a different, and secret, process that bypasses the courts. NSL also include a gag order forbidding the recipient from revealing the existence of the letter to anyone.
Microsoft fought the NSL in question because it violated their policy of notifying all enterprise customers when they receive any "legal order related to data”. The FBI withdrew it without any rulings on the legality or appropriateness of the NSL.
This may indicate a move towards some limitations of the gag order attached to NSLs, which would be very valuable for transparency in the whole process.