Factual error: The treasury officer stated that the couple was from "Las Vegas County". Anyone from the government (local/county/state/federal) should know Las Vegas is in 'Clark County'. Sara (who works for LVMPD/Clark County) did not react at all.
Visible crew/equipment: When Warrick is talking to Greg in the DNA lab comparing Brass's DNA, the boom mic is reflected on the back wall.
Continuity mistake: When Warrick is talking to Dr. Robins, his collar changes place. During the close-ups, only the right side of his collar is tucked in. During shots farther away, both sides are tucked in.
Continuity mistake: When Brass is searching for Ellie's car on his in-car computer, it identifies it as a 1976 Camaro, however when he spots it and calls for back up, he says it's a '78 Camaro. Additionally, the licence plate is also different to what was listed on the computer.
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.