Another idea in the Parsha yesterday is the concept of מודה במקצת, which means a partial admission. This refers to a case where a person is asked to pay back a loan and he denies owing the full amount but only owns a part of it. In such a case, the person is forced to make an oath to confirm that what he's saying his true.
If the person denies the loan entirely, he is entirely exempt and he owes nothing. The rational given in the Talmud, is a lesson in human nature. The Gemara says that a person would not dare deny a loan in its entirety when someone is kind enough to lend him money. But he would try to delay repayment by only admitting part of the loan.
From personal experience, I have seen that the rules regarding מודה במקצת are really true.