Testimony of an unlearned lay-person: Pesachim 49b
Testimony of a Shepherd of others' animals: Bava Metzia 5b
Testimony of a Shepherd of his own animals: Bava Metzia 5b
Testimony of one who falsely denies a loan: Bava Metzia 5b
Testimony of one who falsely denies having been given an object to guard: Bava Metzia 5b
One who is a thief under Rabbinic laws: Rosh HaShanah 22a