How about the 2009 New York City Subway plot[1]? I suppose it's possible that Zazi was a patsy, but if you take the Wikipedia page at face value it seems pretty damning.
[1] https://en.wikipedia.org/wiki/2009_New_York_City_Subway_and_...
[1] https://en.wikipedia.org/wiki/2009_New_York_City_Subway_and_...
To be clear, “patsy” is too harsh a term for what I meant. There is value in diverting the attention of people intending to do harm from real plots to fake ones invented by the FBI. However, that is not “thwarting an attack”, and in my opinion does not justify excessive surveillance.