And Then There Were Fewer - S9-E1
Other mistake: When everyone is leaving the mansion after Stephanie was killed, 3 mistakes happen at the same time: Dr. Hartman came alone, but there is a person sitting in the front seat next to him. Derek is in the car he and Jillian came in when they got here, but he is alone. And Carl was getting into Herbert's Ice Cream Truck, which was in front of his own truck.