Bava Kamma 107: Belief in the Power of Chutzpah

When a person denies robbery completely, he is believed.  When a person admits to part of a robbery, he must pay back the amount admitted to and then take an oath regarding his claim of not owning more.  

Rabat teaches hazakah en adam me'z paan liane baal have  a person will not be so bold to deny his guilt to the faces of those he owes.  He gives the robber the benefit of the doubt: perhaps he'll pay the full sum later.

Rash explains the expression differently. Perhaps when the lender did something so generous - offering a loan, which had no interest - the borrower would not be able to stomach lying to the lender's face.  Others suggest that this ruling applies to circumstances other than loans, meaning that the accused was not always facing a wonderfully generous person.  The borrow simply can't lie in front of someone who knows the truth.