The exception is in which a witness commits the crime of perjury which could be the offence of lying towards the court. In that case, the testimony on the witness could possibly be used to point out that he / she lied in court. This is the quantitative research in https://islamicrulesofinheritance74062.pointblog.net/the-2-minute-rule-for-ghulam-ishaq-khan-institute-of-engineering-sciences-and-technology-88077118