As Frobisher, Lois, and Bridget, seen from behind, start up the stairs together, two other people come down past them. In the reverse shot, when they are less than halfway up, the other people have vanished.
When Gwen is driving onto the Severn Bridge, the POV shot shows her on the bridge but the side shots show trees and hedges next to the car, showing her not to be on the bridge.
The Government Information Network page on Torchwood refers to "HRH Queen Victoria". The Queen is referred to as Her Majesty (HM), not Her Royal Highness (HRH).
Colonel Oduya states that the children stopped at "8.40 GMT". From the paper that Ianto picks up, the date is given as Wednesday, September 2009. If this is correct, then the UK is still on BST (British Summer Time), and 8.40 GMT doesn't fit with the school run.
During the second takeover of the children, a blond boy in a red school uniform (farthest to the right) briefly glances at the camera when all the other children's eyes are fixed off in the distance. When he notices that he's on camera, his eyes quickly dart back to fixed and blank.
When Jack is examining the dead Chinese man in the morgue, the 'body's' chest expands and contracts as he breathes.
After Ianto's brother-in-law lobs a brick at the stolen car, in the background, the brick hits the ground and bounces several times.