to permanently deprive the owner or rightful possessor of that property or its use.

For example, if X goes to a restaurant and, by mistake, takes Y's scarf instead of her own, she has physically deprived Y of the use of the property (which is the actus reus) but the mistake prevents X from forming the mens rea (i.e., because she believes that she is the owner, she is not dishonest and does not intend to deprive the "owner" of it) so no crime has been committed at this point. But if she realises