adlc, to take an example (just one easy one from many many examples), in secular courts the prosecutor can get a conviction by “proving” the accused earned “X” dollars per year based on the testimony of a single “witness” who pled guilty to another crime/case and worked out a prosecutorial agreement for a lesser sentence in exchange for testifying against the accused in another case. If he doesn’t have anyone else to testify a crime against, he doesn’t get a lesser sentence. It is in his interest to “find” crimes he “knows” about and can offer to testify against so that the prosecutor is made happy to advance his career and public perception for fighting crime. Very often such witnesses testify falsely for that lesser sentence.
In a criminal Din Torah case such a “witness” would be disqualified for multiple reasons. Some of those reasons are a) he’s a single witness b) if it’s a she that alone disqualifies c) he’s a convicted criminal, which alone disqualifies him from being a witness.