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.
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)
Answer: Several times during the episode, most notably at the beginning when the old grave is discovered, Tosh holds a devise that looks a great deal like the Sonic Screwdriver that she uses to determine the age of the skeleton. However, a search of Google images shows that it is not the same thing.
OneHappyHusky