Am I the only one who thinks the LF/Cersei confrontation is ok with his character? I've aleays portrayed him as a bit of a gambler, one who plays the game of thrones remarkably well but simply doesn't take it as seriously as he should. At different times, he's cocky, arrogant, patronizing about the others and prone to pinpoint their shortcomings when he thinks he can get away with it - he did it with Ned and Tyrion. Until that moment, Cersei hadn't been specially power drunk with him or anyone, he probably underplayed her importance and her will to use her power, so he thought he could engage her in some funny insult trading like he did with lots of other people before. Also, he wasn't exactly displaying some secret knowledge that was his ace in the sleeve - the cat was out of the hat already because of Stannis.
