One thing I always wonder about is Littlefinger's plan. He seems to be the Joker of Westeros, sowing discord without any real purpose. He undoubtedly knows that Cersei's children are illegitimate, but he keeps that information to himself. Perhaps he wants to have something to hold over Joffrey later. When Jon Arryn finds out, Littlefinger persuades Lysa to kill him. When Ned Stark finds out, Littlefinger betrays him and lets him die. But then Littlefinger himself has Joffrey assassinated later.
(I almost posted that in the other thread. This can get confusing. I would have ruined a lot of people's day...)