def _check_for_unknown_fields(cls, data):
    """
    Checks for keys in a dictionary that don't exist as fields on a model.

    :param cls:
        The ``Model`` class to validate ``data`` against.

    :param data:
        A dict with keys to validate.

    :returns:
        Errors of the fields that were not present in ``cls``.
    """
    errors = {}
    rogues_found = set(data) - set(cls._fields)
    if len(rogues_found) > 0:
        for field_name in rogues_found:
            errors[field_name] = [u'%s is an illegal field.' % field_name]
    return errors