When Johnnie, with his brother beside him, confronts the police superintendent, the window he is standing in front of is open in the shot from behind him. In the next shot from outside the window is closed. Plus, when he fires, the glass doesn't start to immediately break, and the number of holes in the glass does not correspond to the number of shots fired.
Johnny makes the children go from the classroom across the corridor to another one, but another view just afterwards shows him going back into the first classroom, with the door in the glass broken, and all the children are there.
When Cathie is in the classroom she is wearing a polka-dot blouse, but immediately before and after, it is plain.
The submachine gun in the case did not have a magazine and no ammunition with it so the gun could not be fired.
Despite its Liverpool setting not a single character speaks with a Merseyside (Liverpool) accent in the entire film.