The answer is Korra's hair.

Harmonic Convergence, Darkness Falls, and Light in the Dark (the last 3 episodes of Book 2) are probably the most fight heavy episodes in season 2, and Korra is in most of them. Usually when Korra fights, her hair is up in the stiff and unmoving ponytails, which means that they don't have to be that focused on in fight animation. But, in the last 3 episodes, for the most part, her long hair is down when she fights. This caused major problems for animators, because in real life, loose hair (without any hair product) does not stay in place when someone is moving, so they had to animate the hair in a different position in almost every drawing. This was really hard and tedious to do, becoming one of (or the most) difficult thing to draw for the end of season 2. I'm 50% sure that's one of the reasons why Studio Mir needed some help with the final fight of season 3, when Korra's long hair is down again.