When Dr. Horwood visits Liz at her house, she pulls up in her car, which is quite dirty. When they leave, her car is sparkling clean.
When the doctor drops Liz off at her house the top of the car is up, when she pulls up however the roof is down.
When Liz is wailing in the tunnel and a hooded Mike appears, the string pull on her hoodie appears and disappears.
About 40 minutes into the film, Liz points the torch at Mike, they are level, but when the view moves to Liz's point of view it is slanting downward, as if from the ceiling.
In the downward shot as the four enter the shelter, two legs of the camera's tripod can be seen.
British schools have holidays, not "vacations" as in the USA.
During the opening scene where Lizzie calls the police from the school payphone, she mutters the numbers "9-1-1" under her breath as she dials. In the UK, emergency services are reached by dialing 999.
The boys are old enough to have chest hair, but don't grow beards.