If the claimant on the debt is a cheresh / shoteh / Minor: Ketuvot 18a
If the debtor denies the debt, but is established as a denier of debt: Bava Metzia 16b-17a
If the borrower claims I paid" to the court's statement of debt or to the court's order to pay, and witnesses testify that he did not pay, and then he claims "I paid" at a later date: Bava Metzia 17a
Statement of "The loan never occurred" followed by "I paid": Bava Metzia 17a