Children of Earth: Day One - S3-E1
Revealing mistake: When the children are possessed by the 456, they're supposed to be frozen and stare blankly ahead. But during the second possession, as the camera is panning across a school playground, a boy on the far right is looking directly at it. As it moves closer to him, he swivels his eyes back to where they're supposed to be. (00:16:30)
Children of Earth: Day Two - S3-E2
Revealing mistake: When Ianto drops the concrete cell into the quarry and it breaks apart, the green foam from which it is made becomes visible. It is evident that the outside of the block was painted to resemble bricks and concrete block. When this breaks apart on the ground, the inside becomes visible, showing the concrete and brick textures are only surface deep. (00:53:05)
Revealing mistake: From the first "front on" view of Gwen and Owen in the arcade as they are running to catch the alien device, you can see behind Gwen to the right that Owen is being played by a stunt double.
Chosen answer: As long as Jack's been in charge of it, Torchwood Cardiff has had a different agenda than Torchwood One. Tosh and Owen were there to watch for signs of the 10th Doctor, however, it was the 9th that showed up when the ship crashed in London.
Captain Defenestrator