Cases from the mishnah - when a customer asks the shopkeeper for X amount of produce. And then they dispute whether the customer every actually paid for the produce. Plus, a similar case with a money-changer and smaller coins for a dinar -- but did the customer hand over the dinar? Both cases are necessary. Also, orphans inheriting debt/payback from... orphans, instead of the original lender/borrower -- that is, pay close attention to who dies in which order.