[Answered ]-Django Mixing State Between Forms

Figured it out – turns out it was to do with the behaviour of get_initial in 1.3, which was changed in Django 1.4

https://docs.djangoproject.com/en/dev/releases/1.4/#formmixin-get-initial-returns-an-instance-specific-dictionary

We changed our get_initial to use a copy of the dict rather than the default implementation, and now all is well 🙂