The case of vessels or clothing left with a craftsperson, where the customer gets someone else's property in return - that customer can use the other person's property until the error is rectified - unless the error takes place in a private setting. The Gemara goes on to explore the difference in the two cases. Also, the case of the sharecropper - who doesn't have the presumption of ownership that is "chazakah," with the question of why it's not obvious that he doesn't.