Some National Security Agency analysts deliberately ignored restrictions on their authority to spy on Americans multiple times in the past decade, contradicting Obama administration officials and lawmakers statements that no willful violations occurred.
They say one case per year over 10 years. But we've seem that they make thousands of "mistakes" per year, and we've seen that their official instructions are designed to make auditing difficult if not impossible.
It's also important to distinguish between people "not breaking the law" and "not doing anything wrong." The law authorizes surveillance that violates any reasonable reading of the 4th Amendment (and no, the FISA court is not reasonable). That does not mean that NSA employees never did anything wrong.