Excellent question.
Right now, in my beta-ish stage, I am saving all continuations. Yup, allofem. So when they hit 'back' they go back in time to a previous state and can continue from there. Or they can split into two browsers and each get their own branch, etc.
In the future I will add some management stuff for the application to access. The idea is to have a call which says "ok, now that I'm done processing their credit card, delete all the previous states so they can't go back and do it again on accident".