With normal forms, writing
class FooForm(forms.Form):
myfoo = forms.CharField()
form = FooForm()
creates a class where form.myfoo gives an attribute error: the CharField is moved to FooForm.base_fields['myfoo'] by the metaclass and deep-copied to form.fields['myfoo'] by the constructor. With an equivalent ModelForm, this doesn't happen and form.myfoo and form.fields['myfoo'] both refer to a CharField instance.
This would be just a slight incompatibility, except that del(form.fields['myfoo']) should remove the field from the form - this is the intended behaviour for standard Forms. With ModelForms, the spurious attribute means that, in a template, {{form.myfoo}} falls back to displaying the repr of the CharField (eg. <django.newforms.fields.CharField object at 0x1fb0a90>) instead of nothing. To delete fields from a ModelForm you need to write
del(form['myfoo'])
del(form.foo)
