point is its a mistake by the victim, not an act of theft done by the benefactor. theft by definition is done by the person who benefits from it. if you gave a $100 bill as a gift to the wrong person (ex, mixed up envelopes between a friend and a family member), its not theft if you ask for it back and they dont give it to you. thats them being an starfish. its morally wrong, but its not a crime, unless he intentionally deceived you or swapped the envelopes