In the moment when Elsa takes her hair down, her braid passes through her shoulder. See here:
Anyway, a lot of people are saying that it was on purpose and there’s a pretty good quote on that over here:
The shot where [Elsa] lets her hair down becomes [awesome] when you learn that it was nearly impossible to do. The animators had to phase Elsa’s braid through her arm to keep the model from breaking, but you can hardly see it.