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)
Children of Earth: Day Two - S3-E2
Continuity mistake: When Gwen and Rhys are travelling to London in the lorry full of potatoes, Rhys picks up a potato in his right hand. However when the shot changes, he is holding it in his left hand. (00:29:40)
Children of Earth: Day Two - S3-E2
Character mistake: When lanto is in the park and the children all stop the camera shifts to look at a little girl in pink. But in the top left corner there is a young blonde girl still moving and looks to be silently mouthing a word as she steps off the roundabout. (00:32:08)
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