19 Down (1) - S9-E9
Factual error: Season 9, Episode 9 - 19, "Down": Towards the end of the episode they are searching along railroad tracks. A MetroLink train passes by. MetroLink serves the greater Los Angeles area, but does not serve Las Vegas.
Factual error: At the time where they are looking into the moon and the time, they misstate the time the moon rose in 1997 and they state that the moon was waning, when it actually waxed in the middle of June in 1997. (00:33:00)
Factual error: When Nick picks up the contact lens from the PI's body, you can tell from the way it bends while he is holding it with the forceps that it is soft and pliable, as if it had just come out of the eye. However, after 6-12 hours outside of an eye or a contact case (the amount of time David says the PI has been dead), a soft contact will dry out and become wrinkled and hard.
Answer: He doesn't have the authority to arrest him - when anyone is arrested in the show, a police officer does it. Also he tells Ty that he is going to be submitting his case findings to Ty's insurance company, who would no doubt contact the IRS, who then would have him arrested for fraud etc.