def _errant_items(items, allowed_items):
    """Determine if any items are not allowed.

    Args:
        items (list): items to evaluate
        allowed_items (list): the only allowed items

    Returns:
        (set) unique errant values in `items`
    """
    if not isinstance(allowed_items, set):
        allowed_items = set(allowed_items)
    if not isinstance(items, set):
        items = set(items)
    return items.difference(allowed_items)
