I hit an error today where my view was trying to call get_success_url() on ModelFormMixin, with success_url set, but without self.object being present. This will fall back to a static url if self.object isn't available.
modified ModelFormMixin.get_success_url to check for self.object
I found that i wasn't returning the instance at the end of the save() function that I overwrote. disregard