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.
Audio problem: In the episode "In the public interest" there's a scene where Frost and George are in the car. Frost's phone rings and you can hear both the ring tone and sounds of the phone vibrating against a surface. The problem is that when George is holding the phone in his hand it is still making very loud vibrating sounds, like it would if it was still lying on a surface. When you hold a vibrating phone in your hand it doesn't make noises like that.