Exactly the reason for the tenet "ignorance is no excuse."
IMO there are two conspiracies, the FISA one orchestrated by the FBI and the spying aspect done by Brennan, Clapper and the white house. Both agencies were fighting to be the one who brought in Trumps head on a pike.
I was saying this tongue in cheek. But you are right. Feigning ignorance will not work for Clapper.