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.
Factual error: When Gwen finds Toshiko's hidden message from 1941, she realizes that it was written in blood, and her flashlight reveals the bright red letters on the now-yellowed piece of paper. But after 60+ years, the iron in the blood would oxidize. The writing on the note should be rust-brown, not red.
Revealing mistake: 3-2 (Children of Earth: Day 2): 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.
Factual error: "Captain" Jack Harkness actually wears the rank insignia of a group captain, equivalent to an army colonel and never abbreviated to "captain". In fact, the RAF does not have a rank of captain (the equivalent rank is flight lieutenant). Neither does it have the position of "squadron captain", as he is described - a squadron commander in World War II usually held the rank of squadron leader (equivalent to an army major).