You'll Never Get Away From Me - S2-E3
Corrected entry: The polygraph scene was completely fictitious. There is a very good reason why polygraph results are not admissible as evidence in court: Whether you are lying or not is the opinion of the examiner. There is no clear indicator, like the huge spike shown, that the individual is lying. Also, the one being examined is instructed to lie to certain known questions; e.g., age, address, etc. The examiner needs to see how the body reacts when the person is lying.
Correction: The examiner did ask her the control questions in the beginning. For example, he asked if she was married to Rex. This is a matter of public record and therefore a fact (they can check). These controls questions are used to compare the answers of the questions to. However, it's true that the polygraph test isn't 100% reliable, and it can be faked, therefore it's not admissible to court.