Continuity mistake: Mr Mitchell and Carol are in the back seat of the police car looking for Trisha, and when they see her walking home alone they stop to pick her up. In the following scenes of the police car speeding through the streets you can see that Trisha is not in the car, only Carol and Mitchell are in the back seat, but when the police car stops to arrest the man who is following Cathy, Trisha is there again.
Continuity mistake: When Carol meets Trisha at home time at the school's main entrance, behind them you can see two cars parked in front of the school - a black Humber facing the school and a red Vauxhall Victor. When the shot changes to Cathy standing outside the school's front gate you can see the red Victor has gone, the Humber is now parked in the same place but facing away from the school, and a white VW camper has appeared next to it.
Plot hole: In this episode Cathy's father comes looking for her, he walked out on his family eleven years ago and had no contact with them during that time. Cathy's mother always told her that her father had died when she was a baby so A) How did Mr Hargreaves know which school Cathy attended? and B) how did he recognize her straight away when he hadn't seen her since she was a year old baby and she was now twelve years old?