Before 6dffb07 sometimes TestNestedFormsetFieldForm.is_valid() would throw an exception (from FormsetWidget.get_value_from_datadict) that 'prefix' property is not set in FormsetField.
This is really strange since the form mixin assures that the prefix is set.
It would be possible that the prefix is not yet set, and somebody generated the field without the form, but in this case it happened in a simple view where form.is_valid() was called right after form was initialized.
I tried walking though it in the debugger, and it all worked as expected, and I could not reproduce the error if I stepped with it. However if I let it execute the initialization (without stepping in), prefix was still missing.