When an invalid value is present in the database that isn't part of the list of choices (perhaps dealing with a legacy database) a KeyError will be raised in to_representation() of ChoiceField/MultipleChoiceField.

Personally I'd like to see the behaviour more in line with the Django ORM's choices behaviour where the invalid value is still readable, perhaps by simply returning it from to_representation()

In case this is disagreeable, I think the KeyError should be caught and a new exception raised with a more easily debuggable error message explaining which field has the invalid value.