Continuity mistake: Frost bumps into Mullett's car and breaks a brake light. Part of the red casing falls to the floor. When Frost walks over to look at the damage and sweep it away with his foot, a light bulb is now on the floor. (00:09:50)
Mistaken Identity: Part 2 - S9-E2
Revealing mistake: Near the end when Frost calls Pam to warn her about Meadows, she walks in to the hall and picks up the cordless phone. After she's looked out of the window and seen the killer staring at her, she 'does a Kim Bauer' and holds the phone upside down while she continues speaking with Frost. (02:09:00 - 02:10:20)
Desk Sgt.: Mr Mullet wants to see you as soon as you come in.
Frost: I'll let you know when that is.
Trivia: R.D. Wingfield, author of the book "Frost" on which the series is based, hated the show.